C# type narrowing with if condition of nullable int

Question

The compiler complains that int? cannot be converted to int. I come from a typescript background where this would work because the compiler understands that row can't be null. Am I missing something or do I have to explicitly convert row to int with int test = (int) row?

int? row = GetRowOfCode(excel, code);
if (row != null)
{
  int test = row;
}

https://stackoverflow.com/questions/6389437/explanation-of-int-vs-int — Josh Heaps, Jul 19 '22 at 15:50
You can simplify the check with row.HasValue, and to copy the data: int test = row.Value. — emagers, Jul 19 '22 at 15:51

StriplingWarrior · Accepted Answer · 2022-07-19T17:22:53.933

In C#, once a variable is declared, it keeps the type it's declared as.

With reference types, like string, the nullable syntax is only supported by attributes and inference at compile-time, so string? is effectively the same as string besides some compiler hints. This would allow syntax like yours to work if you were working with a reference type.

But with value types, like int, the nullable type is literally a completely different type at run-time, so the value needs to be extracted from an int? in order to treat it like an int. You have to use a completely different variable to hold a strongly-typed int? versus int. That might look something like this:

if (row != null) // or row.HasValue
{
    int test = row.Value;
    // use `test`
}

With newer pattern matching syntax, you can do this instead:

if (row is int test)
{
   // use `test`
}

C# type narrowing with if condition of nullable int

1 Answers1