Integration Services :: Script Task File Check Error

In my SSIS Package I am using Script task to check whether file exists or not.

I am having 3 variable as below

User::V_Filepath >File path
User::V_GeographyLevelFileName >Filename
User::V_FileExistsFlg >Flag

Below is the code I am using for same

String filepath = Dts.Variables["User:V_Filepath"].Value.ToString() + Dts.Variables["User::V_GeographyLevelFileName"].Value.ToString();
if (
File.Exists(filepath)
)
{
Dts.Variables["User::V_FileExistsFlg"].Value = 1;
}
MessageBox.Show(filepath);
MessageBox.Show(Dts.Variables["User::V_FileExistsFlg"].Value.ToString());
Dts.TaskResult = (int)ScriptResults.Success;

But I am getting exception error.

Find the attached screenshot of error ....