From https://www.codeproject.com/Articles/14402/Testing-File-Access-Rights-in-NET

UserFileAccessRights.cs

using System;
using System.Security;
using System.Security.AccessControl;

namespace GCAWeb.Utils
{


	/// <span class="code-SummaryComment"><summary>
	/// Configuring a Web site through a Web interface can be tricky. 
	/// If one is to read and write various files, it is useful to know 
	/// in advance if you have the authority to do so.
	/// 
	/// This class contains a simple answer to a 
	/// potentially complicated question 
	/// "Can I read this file or can I write to this file?"
	/// 
	/// Using the "rule of least privilege", 
	/// one must check not only is access granted but 
	/// is it denied at any point including a possibly recursive check of groups.
	/// 
	/// For this simple check, a look at the user and immediate groups are only checked.
	/// 
	/// This class could be expanded to identify if the applicable allow/deny rule
	/// was explicit or inherited
	/// 
	/// </summary></span>
	public class UserFileAccessRights
	{

		private readonly string _path;
		private readonly System.Security.Principal.WindowsIdentity _principal;

		private readonly bool _denyAppendData = false;
		private readonly bool _denyChangePermissions = false;
		private readonly bool _denyCreateDirectories = false;
		private readonly bool _denyCreateFiles = false;
		private readonly bool _denyDelete = false;
		private readonly bool _denyDeleteSubdirectoriesAndFiles = false;
		private readonly bool _denyExecuteFile = false;
		private readonly bool _denyFullControl = false;
		private readonly bool _denyListDirectory = false;
		private readonly bool _denyModify = false;
		private readonly bool _denyRead = false;
		private readonly bool _denyReadAndExecute = false;
		private readonly bool _denyReadAttributes = false;
		private readonly bool _denyReadData = false;
		private readonly bool _denyReadExtendedAttributes = false;
		private readonly bool _denyReadPermissions = false;
		private readonly bool _denySynchronize = false;
		private readonly bool _denyTakeOwnership = false;
		private readonly bool _denyTraverse = false;
		private readonly bool _denyWrite = false;
		private readonly bool _denyWriteAttributes = false;
		private readonly bool _denyWriteData = false;
		private readonly bool _denyWriteExtendedAttributes = false;
				
		private readonly bool _allowAppendData = false;
		private readonly bool _allowChangePermissions = false;
		private readonly bool _allowCreateDirectories = false;
		private readonly bool _allowCreateFiles = false;
		private readonly bool _allowDelete = false;
		private readonly bool _allowDeleteSubdirectoriesAndFiles = false;
		private readonly bool _allowExecuteFile = false;
		private readonly bool _allowFullControl = false;
		private readonly bool _allowListDirectory = false;
		private readonly bool _allowModify = false;
		private readonly bool _allowRead = false;
		private readonly bool _allowReadAndExecute = false;
		private readonly bool _allowReadAttributes = false;
		private readonly bool _allowReadData = false;
		private readonly bool _allowReadExtendedAttributes = false;
		private readonly bool _allowReadPermissions = false;
		private readonly bool _allowSynchronize = false;
		private readonly bool _allowTakeOwnership = false;
		private readonly bool _allowTraverse = false;
		private readonly bool _allowWrite = false;
		private readonly bool _allowWriteAttributes = false;
		private readonly bool _allowWriteData = false;
		private readonly bool _allowWriteExtendedAttributes = false;

		public bool canAppendData() { return !_denyAppendData && _allowAppendData; }
		public bool canChangePermissions()
		{ return !_denyChangePermissions && _allowChangePermissions; }
		public bool canCreateDirectories()
		{ return !_denyCreateDirectories && _allowCreateDirectories; }
		public bool canCreateFiles() { return !_denyCreateFiles && _allowCreateFiles; }
		public bool canDelete() { return !_denyDelete && _allowDelete; }
		public bool canDeleteSubdirectoriesAndFiles()
		{
			return !_denyDeleteSubdirectoriesAndFiles &&
			       _allowDeleteSubdirectoriesAndFiles;
		}
		public bool canExecuteFile() { return !_denyExecuteFile && _allowExecuteFile; }
		public bool canFullControl() { return !_denyFullControl && _allowFullControl; }
		public bool canListDirectory()
		{ return !_denyListDirectory && _allowListDirectory; }
		public bool canModify() { return !_denyModify && _allowModify; }
		public bool canRead() { return !_denyRead && _allowRead; }
		public bool canReadAndExecute()
		{ return !_denyReadAndExecute && _allowReadAndExecute; }
		public bool canReadAttributes()
		{ return !_denyReadAttributes && _allowReadAttributes; }
		public bool canReadData() { return !_denyReadData && _allowReadData; }
		public bool canReadExtendedAttributes()
		{
			return !_denyReadExtendedAttributes &&
			       _allowReadExtendedAttributes;
		}
		public bool canReadPermissions()
		{ return !_denyReadPermissions && _allowReadPermissions; }
		public bool canSynchronize() { return !_denySynchronize && _allowSynchronize; }
		public bool canTakeOwnership()
		{ return !_denyTakeOwnership && _allowTakeOwnership; }
		public bool canTraverse() { return !_denyTraverse && _allowTraverse; }
		public bool canWrite() { return !_denyWrite && _allowWrite; }
		public bool canWriteAttributes()
		{ return !_denyWriteAttributes && _allowWriteAttributes; }
		public bool canWriteData() { return !_denyWriteData && _allowWriteData; }
		public bool canWriteExtendedAttributes()
		{
			return !_denyWriteExtendedAttributes &&
			       _allowWriteExtendedAttributes;
		}

		/// <span class="code-SummaryComment"><summary>
		/// Simple accessor
		/// </summary></span>
		/// <span class="code-SummaryComment"><returns></returns></span>
		public System.Security.Principal.WindowsIdentity getWindowsIdentity()
		{
			return _principal;
		}
		/// <span class="code-SummaryComment"><summary>
		/// Simple accessor
		/// </summary></span>
		/// <span class="code-SummaryComment"><returns></returns></span>
		public String getPath()
		{
			return _path;
		}

		/// <span class="code-SummaryComment"><summary>
		/// Convenience constructor assumes the current user
		/// </summary></span>
		/// <span class="code-SummaryComment"><param name="path"></param></span>
		public UserFileAccessRights(string path) :
			this(path, System.Security.Principal.WindowsIdentity.GetCurrent())
		{ }


		/// <span class="code-SummaryComment"><summary>
		/// Supply the path to the file or directory and a user or group. 
		/// Access checks are done
		/// during instantiation to ensure we always have a valid object
		/// </summary></span>
		/// <span class="code-SummaryComment"><param name="path"></param></span>
		/// <span class="code-SummaryComment"><param name="principal"></param></span>
		public UserFileAccessRights(string path,
			System.Security.Principal.WindowsIdentity principal)
		{
			this._path = path;
			this._principal = principal;

			try
			{
				System.IO.FileInfo fi = new System.IO.FileInfo(_path);
				AuthorizationRuleCollection acl = fi.GetAccessControl().GetAccessRules
					(true, true, typeof(System.Security.Principal.SecurityIdentifier));
				for (int i = 0; i < acl.Count; i++)
				{
					System.Security.AccessControl.FileSystemAccessRule rule =
						(System.Security.AccessControl.FileSystemAccessRule)acl[i];
					if (_principal.User.Equals(rule.IdentityReference))
					{
						if (System.Security.AccessControl.AccessControlType.Deny.Equals
							(rule.AccessControlType))
						{
							if (contains(FileSystemRights.AppendData, rule))
								_denyAppendData = true;
							if (contains(FileSystemRights.ChangePermissions, rule))
								_denyChangePermissions = true;
							if (contains(FileSystemRights.CreateDirectories, rule))
								_denyCreateDirectories = true;
							if (contains(FileSystemRights.CreateFiles, rule))
								_denyCreateFiles = true;
							if (contains(FileSystemRights.Delete, rule))
								_denyDelete = true;
							if (contains(FileSystemRights.DeleteSubdirectoriesAndFiles,
								rule)) _denyDeleteSubdirectoriesAndFiles = true;
							if (contains(FileSystemRights.ExecuteFile, rule))
								_denyExecuteFile = true;
							if (contains(FileSystemRights.FullControl, rule))
								_denyFullControl = true;
							if (contains(FileSystemRights.ListDirectory, rule))
								_denyListDirectory = true;
							if (contains(FileSystemRights.Modify, rule))
								_denyModify = true;
							if (contains(FileSystemRights.Read, rule)) _denyRead = true;
							if (contains(FileSystemRights.ReadAndExecute, rule))
								_denyReadAndExecute = true;
							if (contains(FileSystemRights.ReadAttributes, rule))
								_denyReadAttributes = true;
							if (contains(FileSystemRights.ReadData, rule))
								_denyReadData = true;
							if (contains(FileSystemRights.ReadExtendedAttributes, rule))
								_denyReadExtendedAttributes = true;
							if (contains(FileSystemRights.ReadPermissions, rule))
								_denyReadPermissions = true;
							if (contains(FileSystemRights.Synchronize, rule))
								_denySynchronize = true;
							if (contains(FileSystemRights.TakeOwnership, rule))
								_denyTakeOwnership = true;
							if (contains(FileSystemRights.Traverse, rule))
								_denyTraverse = true;
							if (contains(FileSystemRights.Write, rule)) _denyWrite = true;
							if (contains(FileSystemRights.WriteAttributes, rule))
								_denyWriteAttributes = true;
							if (contains(FileSystemRights.WriteData, rule))
								_denyWriteData = true;
							if (contains(FileSystemRights.WriteExtendedAttributes, rule))
								_denyWriteExtendedAttributes = true;
						}
						else if (System.Security.AccessControl.AccessControlType.
							Allow.Equals(rule.AccessControlType))
						{
							if (contains(FileSystemRights.AppendData, rule))
								_allowAppendData = true;
							if (contains(FileSystemRights.ChangePermissions, rule))
								_allowChangePermissions = true;
							if (contains(FileSystemRights.CreateDirectories, rule))
								_allowCreateDirectories = true;
							if (contains(FileSystemRights.CreateFiles, rule))
								_allowCreateFiles = true;
							if (contains(FileSystemRights.Delete, rule))
								_allowDelete = true;
							if (contains(FileSystemRights.DeleteSubdirectoriesAndFiles,
								rule)) _allowDeleteSubdirectoriesAndFiles = true;
							if (contains(FileSystemRights.ExecuteFile, rule))
								_allowExecuteFile = true;
							if (contains(FileSystemRights.FullControl, rule))
								_allowFullControl = true;
							if (contains(FileSystemRights.ListDirectory, rule))
								_allowListDirectory = true;
							if (contains(FileSystemRights.Modify, rule))
								_allowModify = true;
							if (contains(FileSystemRights.Read, rule)) _allowRead = true;
							if (contains(FileSystemRights.ReadAndExecute, rule))
								_allowReadAndExecute = true;
							if (contains(FileSystemRights.ReadAttributes, rule))
								_allowReadAttributes = true;
							if (contains(FileSystemRights.ReadData, rule))
								_allowReadData = true;
							if (contains(FileSystemRights.ReadExtendedAttributes, rule))
								_allowReadExtendedAttributes = true;
							if (contains(FileSystemRights.ReadPermissions, rule))
								_allowReadPermissions = true;
							if (contains(FileSystemRights.Synchronize, rule))
								_allowSynchronize = true;
							if (contains(FileSystemRights.TakeOwnership, rule))
								_allowTakeOwnership = true;
							if (contains(FileSystemRights.Traverse, rule))
								_allowTraverse = true;
							if (contains(FileSystemRights.Write, rule))
								_allowWrite = true;
							if (contains(FileSystemRights.WriteAttributes, rule))
								_allowWriteAttributes = true;
							if (contains(FileSystemRights.WriteData, rule))
								_allowWriteData = true;
							if (contains(FileSystemRights.WriteExtendedAttributes, rule))
								_allowWriteExtendedAttributes = true;
						}
					}
				}

				System.Security.Principal.IdentityReferenceCollection groups = _principal.Groups;
				for (int j = 0; j < groups.Count; j++)
				{
					for (int i = 0; i < acl.Count; i++)
					{
						System.Security.AccessControl.FileSystemAccessRule rule =
							(System.Security.AccessControl.FileSystemAccessRule)acl[i];
						if (groups[j].Equals(rule.IdentityReference))
						{
							if (System.Security.AccessControl.AccessControlType.
								Deny.Equals(rule.AccessControlType))
							{
								if (contains(FileSystemRights.AppendData, rule))
									_denyAppendData = true;
								if (contains(FileSystemRights.ChangePermissions, rule))
									_denyChangePermissions = true;
								if (contains(FileSystemRights.CreateDirectories, rule))
									_denyCreateDirectories = true;
								if (contains(FileSystemRights.CreateFiles, rule))
									_denyCreateFiles = true;
								if (contains(FileSystemRights.Delete, rule))
									_denyDelete = true;
								if (contains(FileSystemRights.
									DeleteSubdirectoriesAndFiles, rule))
									_denyDeleteSubdirectoriesAndFiles = true;
								if (contains(FileSystemRights.ExecuteFile, rule))
									_denyExecuteFile = true;
								if (contains(FileSystemRights.FullControl, rule))
									_denyFullControl = true;
								if (contains(FileSystemRights.ListDirectory, rule))
									_denyListDirectory = true;
								if (contains(FileSystemRights.Modify, rule))
									_denyModify = true;
								if (contains(FileSystemRights.Read, rule))
									_denyRead = true;
								if (contains(FileSystemRights.ReadAndExecute, rule))
									_denyReadAndExecute = true;
								if (contains(FileSystemRights.ReadAttributes, rule))
									_denyReadAttributes = true;
								if (contains(FileSystemRights.ReadData, rule))
									_denyReadData = true;
								if (contains(FileSystemRights.
									ReadExtendedAttributes, rule))
									_denyReadExtendedAttributes = true;
								if (contains(FileSystemRights.ReadPermissions, rule))
									_denyReadPermissions = true;
								if (contains(FileSystemRights.Synchronize, rule))
									_denySynchronize = true;
								if (contains(FileSystemRights.TakeOwnership, rule))
									_denyTakeOwnership = true;
								if (contains(FileSystemRights.Traverse, rule))
									_denyTraverse = true;
								if (contains(FileSystemRights.Write, rule))
									_denyWrite = true;
								if (contains(FileSystemRights.WriteAttributes, rule))
									_denyWriteAttributes = true;
								if (contains(FileSystemRights.WriteData, rule))
									_denyWriteData = true;
								if (contains(FileSystemRights.
									WriteExtendedAttributes, rule))
									_denyWriteExtendedAttributes = true;
							}
							else if (System.Security.AccessControl.AccessControlType.
								Allow.Equals(rule.AccessControlType))
							{
								if (contains(FileSystemRights.AppendData, rule))
									_allowAppendData = true;
								if (contains(FileSystemRights.ChangePermissions, rule))
									_allowChangePermissions = true;
								if (contains(FileSystemRights.CreateDirectories, rule))
									_allowCreateDirectories = true;
								if (contains(FileSystemRights.CreateFiles, rule))
									_allowCreateFiles = true;
								if (contains(FileSystemRights.Delete, rule))
									_allowDelete = true;
								if (contains(FileSystemRights.
									DeleteSubdirectoriesAndFiles, rule))
									_allowDeleteSubdirectoriesAndFiles = true;
								if (contains(FileSystemRights.ExecuteFile, rule))
									_allowExecuteFile = true;
								if (contains(FileSystemRights.FullControl, rule))
									_allowFullControl = true;
								if (contains(FileSystemRights.ListDirectory, rule))
									_allowListDirectory = true;
								if (contains(FileSystemRights.Modify, rule))
									_allowModify = true;
								if (contains(FileSystemRights.Read, rule))
									_allowRead = true;
								if (contains(FileSystemRights.ReadAndExecute, rule))
									_allowReadAndExecute = true;
								if (contains(FileSystemRights.ReadAttributes, rule))
									_allowReadAttributes = true;
								if (contains(FileSystemRights.ReadData, rule))
									_allowReadData = true;
								if (contains(FileSystemRights.
									ReadExtendedAttributes, rule))
									_allowReadExtendedAttributes = true;
								if (contains(FileSystemRights.ReadPermissions, rule))
									_allowReadPermissions = true;
								if (contains(FileSystemRights.Synchronize, rule))
									_allowSynchronize = true;
								if (contains(FileSystemRights.TakeOwnership, rule))
									_allowTakeOwnership = true;
								if (contains(FileSystemRights.Traverse, rule))
									_allowTraverse = true;
								if (contains(FileSystemRights.Write, rule))
									_allowWrite = true;
								if (contains(FileSystemRights.WriteAttributes, rule))
									_allowWriteAttributes = true;
								if (contains(FileSystemRights.WriteData, rule))
									_allowWriteData = true;
								if (contains(FileSystemRights.WriteExtendedAttributes,
									rule)) _allowWriteExtendedAttributes = true;
							}
						}
					}
				}
			}
			catch (Exception e)
			{
				//Deal with IO exceptions if you want
				throw e;
			}
		}

		/// <span class="code-SummaryComment"><summary>
		/// Simply displays all allowed rights
		/// 
		/// Useful if say you want to test for write access and find
		/// it is false;
		/// <xmp>
		/// UserFileAccessRights rights = new UserFileAccessRights(txtLogPath.Text);
		/// System.IO.FileInfo fi = new System.IO.FileInfo(txtLogPath.Text);
		/// if (rights.canWrite() && rights.canRead()) {
		///     lblLogMsg.Text = "R/W access";
		/// } else {
		///     if (rights.canWrite()) {
		///        lblLogMsg.Text = "Only Write access";
		///     } else if (rights.canRead()) {
		///         lblLogMsg.Text = "Only Read access";
		///     } else {
		///         lblLogMsg.CssClass = "error";
		///         lblLogMsg.Text = rights.ToString()
		///     }
		/// }
		/// </xmp>
		/// 
		/// </summary></span>
		/// <span class="code-SummaryComment"><returns></returns></span>
		public override String ToString()
		{
			//string str = "";
			System.Text.StringBuilder str = new System.Text.StringBuilder();

			if (canAppendData())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("AppendData");
			}
			if (canChangePermissions())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ChangePermissions");
			}
			if (canCreateDirectories())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("CreateDirectories");
			}
			if (canCreateFiles())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("CreateFiles");
			}
			if (canDelete())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("Delete");
			}
			if (canDeleteSubdirectoriesAndFiles())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("DeleteSubdirectoriesAndFiles");
			}
			if (canExecuteFile())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ExecuteFile");
			}
			if (canFullControl())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("FullControl");
			}
			if (canListDirectory())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ListDirectory");
			}
			if (canModify())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("Modify");
			}
			if (canRead())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("Read");
			}
			if (canReadAndExecute())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ReadAndExecute");
			}
			if (canReadAttributes())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ReadAttributes");
			}
			if (canReadData())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ReadData");
			}
			if (canReadExtendedAttributes())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ReadExtendedAttributes");
			}
			if (canReadPermissions())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("ReadPermissions");
			}
			if (canSynchronize())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("Synchronize");
			}
			if (canTakeOwnership())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("TakeOwnership");
			}
			if (canTraverse())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("Traverse");
			}
			if (canWrite())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("Write");
			}
			if (canWriteAttributes())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("WriteAttributes");
			}
			if (canWriteData())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("WriteData");
			}
			if (canWriteExtendedAttributes())
			{
				if (str.Length > 0) str.Append(",");
				str.Append("WriteExtendedAttributes");
			}
			if (str.Length > 0)
				str.Append("None");
			return str.ToString();
		}

		/// <span class="code-SummaryComment"><summary>
		/// Convenience method to test if the right exists within the given rights
		/// </summary></span>
		/// <span class="code-SummaryComment"><param name="right"></param></span>
		/// <span class="code-SummaryComment"><param name="rule"></param></span>
		/// <span class="code-SummaryComment"><returns></returns></span>
		public bool contains(System.Security.AccessControl.FileSystemRights right,
			System.Security.AccessControl.FileSystemAccessRule rule)
		{
			return (((int)right & (int)rule.FileSystemRights) == (int)right);
		}


		public static string FindFirstFailingPath(string path)
		{
			// Could use bisect to improve performance.

			var chars = new char[] { '\\', '/' };
			var segments = path.Split(chars);

			var aggregate = "";
			foreach (var segment in segments)
			{
				aggregate += segment + '\\';
				try
				{
					var dummy = new UserFileAccessRights(aggregate);
				}
				catch (Exception e)
				{
					break; // Found failing path
				}
			}

			return aggregate;
		}
	}
}