How can i hide MessageBox?

Good evening,
I m trying to hide messageboxes by using WhenCalled method but I encoutnered build error. Would you tell me if you know the solution?

Here is my code
-------------------------------------
[Test]
public void ReadCsvData_OK_RetrievalCsvDataFailed()
{
Isolate.WhenCalled(()=> MessageBox.Show(String.Empty)).WillReturn(DialogResult.OK);

fileTest file = new fileTest();

bool bRtn =file.GetData(FilePath);

Assert.AreEqual(false, bRtn);
}
-------------------------------------
errors is from "WhenCalled(()=> MessageBox" parts.

')' is invalid ,
'>' is invalid
')' is required
; is required

Thak you very much.