c# - Cast object containing int to float results in InvalidCastException

Question

protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
    var fP18VaR = (float) (int)e.Values[0];
}

I'm getting

InvalidCastException - Specified cast is not valid

Why doesn't it work?

The value of e.Values[0] is: 6 666,00

Answer 1

The problem you're running into here is that the C# cast operator means different things in different situations.

Take this example you gave:

object num = 10;
float fnum = (float)num;

The C# compiler will think you are telling it this: "The variable num refers to a boxed float; please unbox it and return the boxed value."

You're getting an error because it's not a boxed float, it's a boxed int.

The problem is that C# uses identical-looking syntax for two totally unrelated operations: 'unbox' and 'numeric conversion'. Here's an example of where a cast means numeric conversion:

int num = 10;
float fnum = (float)num;

Almost exactly the same code, and yet this won't give you an error. And that's because the C# compiler treats this completely differently - this code means: "Please perform a numeric conversion, converting the integer stored in 'num' into a single-precision floating point value."

How do you know which of these two utterly unrelated operations it's going to choose? It's all about the source and destination types. If you're converting from 'object' to a value type, that will always be treated as an unbox. If you're converting from one numeric type to another, that will always be treated as a numeric conversion.

So how do you get the result you want? Well, you need to do both operations: you need to unbox the int and then you need to convert it to a float. So you actually need two casts:

object num = 10;
float fnum = (float) (int)num; 

Horrible huh?

The simplest way to do what you want here is to avoid casting entirely. Just do this:

float fnum = Convert.ToSingle(num); 

That will coerce the type to a single-precision float if it's possible to do so.