Question 135281 in Visual Studio Code Question posted in
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Visual Studio Code – C#: looping over int for IEnumerable<double> causes no casting error

The following code causes CS0266 in Visual Studio:

double x = 1.23;
int y = x;

But the following code compiles in Visual Studio, and causes an implicit cast double to int:

double x = 0;
ReadOnlyCollection<double> y = new ReadOnlyCollection<double>(new double[3] { 1.23, 2.34, 3.45 });
foreach (int z in y)
{
   x += z;
}

Why is this treated differently? Can I cause compilation to fail?

I expect that an implicit cast to int when looping over an IEnumerable, would cause the same error as when casting a double to an int.

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2

Answers


  1. 0

    A foreach loop has a builtin explicit cast to the target type.

    That’s why you can write:

    object[] things = ...

foreach(string s in things)
{
    // ...
}

    You can see this in the C# language spec, 8.8.4.:

    8.8.4 The foreach statement

    foreach (V v in x) embedded-statement

    is then expanded to:

    E e = ((C)(x)).GetEnumerator();
try {
    V v;
    while (e.MoveNext()) {
        v = (V)(T)e.Current;
        embedded-statement
    }
}
finally {
    … // Dispose e
}

    So it works in the same way as if you’d write:

    double d = 123.45;
int i = (int) d;
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  2. 0

    checking you code,

    //here you need a cast
double x = 1.23;
int y = (int)x; //explicit cast

    But in the loop

    int x = 0;
ReadOnlyCollection<double> y = new ReadOnlyCollection<double>(new double[3] {1.23, 2.34, 3.45 });
// the z variable is to get the int in the y list, like a hidden cast
foreach (int z in y)
{
   x += z;
}

    Best Regards

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