CLR Function and Full Trust Question?

The Developer is attempting to run this code:
using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
public partial class UserDefinedFunctions
{
[Microsoft.SqlServer.Server.SqlFunction]
public static SqlString Function2(string str)
{
System.ComponentModel.MaskedTextProvider mm = new System.ComponentModel.MaskedTextProvider("0000");

// Put your code here
str = "sdfs fsdf sdf sdf sdf sdf sdf sdf" + str;
return str;
}
};
When he does, he gets this error message:
Msg 6522, Level 16, State 2, Line 1
A .NET Framework error occurred during execution of user defined routine or aggregate 'Function2':
System.Security.HostProtectionException: Attempted to perform an operation that was forbidden by the CLR host.
The protected resources (only available with full trust) were: All
The demanded resources were: SharedState
System.Security.HostProtectionException:
at System.Security.CodeAccessSecurityEngine.ThrowSecurityException(Assembly asm, PermissionSet granted, PermissionSet refused, RuntimeMethodHandle rmh, SecurityAction action, Object demand, IPermission permThatFailed)
at System.Security.CodeAccessSecurityEngine.ThrowSecurityException(Object assemblyOrString, PermissionSet granted, PermissionSet refused, RuntimeMethodHandle rmh, SecurityAction action, Object demand, IPermission permThatFailed)
at System.Security.CodeAccessSecurityEngine.CheckSetHelper(PermissionSet grants, PermissionSet refused, PermissionSet demands, RuntimeMethodHandle rmh, Object assemblyOrString, SecurityAction action, Boolean throwException)
at System.Security.CodeAccessSecurityEngine.CheckSetHelper(CompressedStack cs, PermissionSet grants, PermissionSet refused, PermissionSet demands, RuntimeMethodHandle rmh, Assembly asm, SecurityAction action)
at UserDefinedFunctions.Function2(String str)

Is there an option that I must turn on so that the operation IS NOT forbidden by the CLR host? That there is full trust?
When the Developer runs this code not in a CLR function it works. When he places it into a CLR function it does not work.

It didn't work. Thank you for pointing me into the correct direction. At least now I am not as lost as I felt before. I will continue to research and if I have further questions I will post them.

It worked when we set PERMISSION_SET to UNSAFE. EXTERNAL_ACCESS did not work for us.