Question

I can’t understand why this won’t work

decimal? compRetAmount = !string.IsNullOrEmpty(txtLineCompRetAmt.Text) 
    ? decimal.Parse(txtLineCompRetAmt.Text.Replace(",","")) 
    : null;

Answer 1

Because null is of type object (effectively untyped) and you need to assign it to a typed object.

This should work:

decimal? compRetAmount = !string.IsNullOrEmpty(txtLineCompRetAmt.Text) 
         ? decimal.Parse(txtLineCompRetAmt.Text.Replace(",","")) 
         : (decimal?)null;

or this is a bit better:

decimal? compRetAmount = !string.IsNullOrEmpty(txtLineCompRetAmt.Text) 
         ? decimal.Parse(txtLineCompRetAmt.Text.Replace(",","")) 
         : default(decimal?);

Here is the MSDN link for the default keyword.