Eternal File.Exist CLR C # does not work when called from SQL Server, but works in C #

I use the CLR to check if a file exists using the C # method. I created the assembly and function in the sql server and I pass the parameter.

C # code:

using System;
using System.IO;

public class FileExist
{
    [Microsoft.SqlServer.Server.SqlFunction]
    public static bool fn_FileExistCLR(string FilePath){     

     return System.IO.File.Exists(FilePath);                
  }
}

I created the assembly on sql server:

CREATE ASSEMBLY FileExistCLR2
FROM 'C:\MSSQLTips\FileExist2.dll'
WITH PERMISSION_SET = EXTERNAL_ACCESS

I also created a function:

CREATE FUNCTION [dbo].[fc_ChkFlCLR2](@filename [nvarchar](4000))
RETURNS nvarchar(4000) WITH EXECUTE AS CALLER
AS 
     EXTERNAL NAME [FileExistCLR2].[FileExist].[fn_FileExistCLR] 
GO

The parameter is passed by the function to the CLR.

declare @filevar as nvarchar(4000)
set @filevar = ltrim(rtrim('T:\\Project\\XML_ErrorFile20131106.txt'))
--set @filevar = ltrim(rtrim('C:\QA_Ticket.xls'))
SELECT [PlayNet].[dbo].[fc_ChkFlCLRChk] (@filevar)   
GO

The local file is located, but not for the file in the network folder:

Result 1 - if the file is in a network folder, the function returns false Result 2 - if the file is in a local folder, the function returns TRUE.

I run them on a local sql server and try to add @, but to no avail.

Cross check. I created a C # method that displays the result on the command line. Curiously, the same file on the network is detected by file.exist. Please, help

using System;
using System.IO;

class FileExist
{
   // [Microsoft.SqlServer.Server.SqlFunction]
    static void Main()
        {
        string FilePath = "T:\\Project\\XML_ErrorFile20131106.txt";
        string retFilePath;
            if (File.Exists(FilePath))
            {
                 retFilePath = "True";
                 Console.WriteLine(retFilePath);
                    Console.ReadLine();
            }
            else 
                retFilePath = "false";
                Console.WriteLine(retFilePath);
                    Console.ReadLine();
        }
}