Do not use OfType<T>() with impossible types

[terrajobst]

Warn if use of OfType<T>() would provably result in an empty sequence, because we can see that the input type in the sequence will never be the specified type.

Category: Reliability
Suggested severity: Warning

UPDATE FROM roslyn-analyzer issue :

The following will always throw an InvalidCastException at runtime:

new int[] { 1 }.Cast<string>().ToList();
new float[] { 1f }.Cast<double>().ToList();
new int[] { 1 }.Cast<long>().ToList();

Perhaps worse, this will return an enumerable that fully enumerates the source and then returns no items silently:

enumerableOfApple.OfType<Orange>()

Both of these situations are likely to be a mistake.

Ther is a PR out for review

[jeffhandley]

Estimates:

• Analyzer: Small
• Fixer: Not Applicable

[drewnoakes]

Possibly at the same time, though via a different diagnostic ID: calling OfType<T>() on an IEnumerable<T> where the T is identical or assignable is redundant.

Enumerable.Range(1, 100).OfType<int>();

// @jnyrup points out that this is actually a non-null check,
// so isn't completely safe to remove

// myString.Split(",").OfType<object>();

[fowl2]

I did a little prototype for fun over at dotnet/roslyn-analyzers#3608

[jnyrup]

@drewnoakes Note that for reference types this can be used as a less clear way of writingWhere(e => e != null).

[bartonjs]

Video

Approved as proposed

Category: Reliability
Suggested severity: Warning