Validating a numeric only textbox

I've created a form based program that need some validation, all I need to to make sure the user can only enter numeric value in the distance text field

So far, I've checked that it's got something in it, if it has something then go on to validate it's numeric

else if (txtEvDistance.Text.Length == 0)
         {
             MessageBox.Show("Please enter the distance");
         }
 else if (cboAddEvent.Text //is numeric)
         {
             MessageBox.Show("Please enter a valid numeric distance");
         }
 

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