Difference in behavior from MS and Mono's C# compilers

gistfile1.cs

/**
C:\Development>mcs dev.cs
dev.cs(8,18): warning CS0649: Field `WindowsIdentity._type' is never assigned to, and will always have its default value `null'
Compilation succeeded - 1 warning(s)

C:\Development>csc /nologo dev.cs
dev.cs(14,17): error CS0546: 'WindowsIdentity.AuthenticationType.set': cannot override because 'ClaimsIdentity.AuthenticationType' does not have an
        overridable set accessor
dev.cs(3,24): (Location of symbol related to previous error)
*/

public class ClaimsIdentity
{
	public virtual string AuthenticationType { get; private set; }	
}

public class WindowsIdentity : ClaimsIdentity
{
		private string _type;
	
		// properties
//#if NET_4_5
		sealed override
//#endif
		public string AuthenticationType {
			get { return _type; }
		}	
}
					
public class Program
{
	public static void Main()
	{
	}
}