Why can’t I assign null to decimal with ternary operator?

I can’t understand why this won’t work

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

Answers:

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Method 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.

Method 2

Don’t use decimal.Parse.

Convert.ToDecimal will return 0 if it is given a null string. decimal.Parse will throw an ArgumentNullException if the string you want to parse is null.

Method 3

Try this:

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

The problem is that the compiler does not know what type nullhas. So you can just cast it to decimal?

Method 4

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

Method 5

Because the compiler can’t infer the best type from the operands of the conditional operator.

When you write condition ? a : b, there must be an implicit conversion from the type of a to the type of b, or from the type of b to the type of a. The compiler will then infer the type of the whole expression as the target type of this conversion. The fact that you assign it to a variable of type decimal? is never considered by the compiler. In your case, the types of a and b are decimal and some unknown reference or nullable type. The compiler can’t guess what you mean, so you need to help it:

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

Method 6

You need to cast the first part to decimal?

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

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