Using the Windows Credential API (CredRead, CredWrite, CredDelete, CredEnumerate).

CredentialManager.cs

// The most up to date version is available 
// on GitHub: https://github.com/meziantou/Meziantou.Framework/tree/master/src/Meziantou.Framework.Win32.CredentialManager
// NuGet package: https://www.nuget.org/packages/Meziantou.Framework.Win32.CredentialManager/

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Text;
using Microsoft.Win32.SafeHandles;

public static class CredentialManager
{
    public static Credential ReadCredential(string applicationName)
    {
        IntPtr nCredPtr;
        bool read = CredRead(applicationName, CredentialType.Generic, 0, out nCredPtr);
        if (read)
        {
            using (CriticalCredentialHandle critCred = new CriticalCredentialHandle(nCredPtr))
            {
                CREDENTIAL cred = critCred.GetCredential();
                return ReadCredential(cred);
            }
        }

        return null;
    }

    private static Credential ReadCredential(CREDENTIAL credential)
    {
        string applicationName = Marshal.PtrToStringUni(credential.TargetName);
        string userName = Marshal.PtrToStringUni(credential.UserName);
        string secret = null;
        if (credential.CredentialBlob != IntPtr.Zero)
        {
            secret = Marshal.PtrToStringUni(credential.CredentialBlob, (int)credential.CredentialBlobSize / 2);
        }

        return new Credential(credential.Type, applicationName, userName, secret);
    }

    public static void WriteCredential(string applicationName, string userName, string secret)
    {
        byte[] byteArray = secret == null ? null : Encoding.Unicode.GetBytes(secret);
        // XP and Vista: 512; 
        // 7 and above: 5*512
        if (Environment.OSVersion.Version < new Version(6, 1) /* Windows 7 */)
        {
            if (byteArray != null && byteArray.Length > 512)
                throw new ArgumentOutOfRangeException("secret", "The secret message has exceeded 512 bytes.");
        }
        else
        {
            if (byteArray != null && byteArray.Length > 512 * 5)
                throw new ArgumentOutOfRangeException("secret", "The secret message has exceeded 2560 bytes.");
        }

        CREDENTIAL credential = new CREDENTIAL();
        credential.AttributeCount = 0;
        credential.Attributes = IntPtr.Zero;
        credential.Comment = IntPtr.Zero;
        credential.TargetAlias = IntPtr.Zero;
        credential.Type = CredentialType.Generic;
        credential.Persist = (uint)CredentialPersistence.LocalMachine;
        credential.CredentialBlobSize = (uint)(byteArray == null ? 0 : byteArray.Length);
        credential.TargetName = Marshal.StringToCoTaskMemUni(applicationName);
        credential.CredentialBlob = Marshal.StringToCoTaskMemUni(secret);
        credential.UserName = Marshal.StringToCoTaskMemUni(userName ?? Environment.UserName);

        bool written = CredWrite(ref credential, 0);
        Marshal.FreeCoTaskMem(credential.TargetName);
        Marshal.FreeCoTaskMem(credential.CredentialBlob);
        Marshal.FreeCoTaskMem(credential.UserName);

        if (!written)
        {
            int lastError = Marshal.GetLastWin32Error();
            throw new Exception(string.Format("CredWrite failed with the error code {0}.", lastError));
        }
    }

    public static IReadOnlyList<Credential> EnumerateCrendentials()
    {
        List<Credential> result = new List<Credential>();

        int count;
        IntPtr pCredentials;
        bool ret = CredEnumerate(null, 0, out count, out pCredentials);
        if (ret)
        {
            for (int n = 0; n < count; n++)
            {
                IntPtr credential = Marshal.ReadIntPtr(pCredentials, n * Marshal.SizeOf(typeof(IntPtr)));
                result.Add(ReadCredential((CREDENTIAL)Marshal.PtrToStructure(credential, typeof(CREDENTIAL))));
            }
        }
        else
        {
            int lastError = Marshal.GetLastWin32Error();
            throw new Win32Exception(lastError);
        }

        return result;
    }

    [DllImport("Advapi32.dll", EntryPoint = "CredReadW", CharSet = CharSet.Unicode, SetLastError = true)]
    static extern bool CredRead(string target, CredentialType type, int reservedFlag, out IntPtr credentialPtr);

    [DllImport("Advapi32.dll", EntryPoint = "CredWriteW", CharSet = CharSet.Unicode, SetLastError = true)]
    static extern bool CredWrite([In] ref CREDENTIAL userCredential, [In] UInt32 flags);

    [DllImport("advapi32", SetLastError = true, CharSet = CharSet.Unicode)]
    static extern bool CredEnumerate(string filter, int flag, out int count, out IntPtr pCredentials);

    [DllImport("Advapi32.dll", EntryPoint = "CredFree", SetLastError = true)]
    static extern bool CredFree([In] IntPtr cred);

    private enum CredentialPersistence : uint
    {
        Session = 1,
        LocalMachine,
        Enterprise
    }

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
    private struct CREDENTIAL
    {
        public uint Flags;
        public CredentialType Type;
        public IntPtr TargetName;
        public IntPtr Comment;
        public System.Runtime.InteropServices.ComTypes.FILETIME LastWritten;
        public uint CredentialBlobSize;
        public IntPtr CredentialBlob;
        public uint Persist;
        public uint AttributeCount;
        public IntPtr Attributes;
        public IntPtr TargetAlias;
        public IntPtr UserName;
    }

    sealed class CriticalCredentialHandle : CriticalHandleZeroOrMinusOneIsInvalid
    {
        public CriticalCredentialHandle(IntPtr preexistingHandle)
        {
            SetHandle(preexistingHandle);
        }

        public CREDENTIAL GetCredential()
        {
            if (!IsInvalid)
            {
                CREDENTIAL credential = (CREDENTIAL)Marshal.PtrToStructure(handle, typeof(CREDENTIAL));
                return credential;
            }

            throw new InvalidOperationException("Invalid CriticalHandle!");
        }

        protected override bool ReleaseHandle()
        {
            if (!IsInvalid)
            {
                CredFree(handle);
                SetHandleAsInvalid();
                return true;
            }

            return false;
        }
    }
}

public enum CredentialType
{
    Generic = 1,
    DomainPassword,
    DomainCertificate,
    DomainVisiblePassword,
    GenericCertificate,
    DomainExtended,
    Maximum,
    MaximumEx = Maximum + 1000,
}

public class Credential
{
    private readonly string _applicationName;
    private readonly string _userName;
    private readonly string _password;
    private readonly CredentialType _credentialType;

    public CredentialType CredentialType
    {
        get { return _credentialType; }
    }

    public string ApplicationName
    {
        get { return _applicationName; }
    }

    public string UserName
    {
        get { return _userName; }
    }

    public string Password
    {
        get { return _password; }
    }

    public Credential(CredentialType credentialType, string applicationName, string userName, string password)
    {
        _applicationName = applicationName;
        _userName = userName;
        _password = password;
        _credentialType = credentialType;
    }

    public override string ToString()
    {
        return string.Format("CredentialType: {0}, ApplicationName: {1}, UserName: {2}, Password: {3}", CredentialType, ApplicationName, UserName, Password);
    }
}

Program.cs

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(CredentialManager.ReadCredential("Demo"));
        CredentialManager.WriteCredential("Demo", "Meziantou", "Passw0rd");
        CredentialManager.WriteCredential("http://www.softfluent.com", "Meziantou", "Passw0rd");
        Console.WriteLine(CredentialManager.ReadCredential("Demo"));

        foreach (var credential in CredentialManager.EnumerateCrendentials())
        {
            Console.WriteLine(credential);
        }
    }
}

Is there a reason you are using Marshal.StringToCoTaskMemUni on the target, username, and password. I created a similar function that does not use that and just passes the strings in from managed memory. Instead pointers I use strings for the username/pw/target.