What's the strangest corner case you've seen in C# or .NET? [closed]

Answer 1

This is the strangest I've encountered by accident:

public class DummyObject
{
    public override string ToString()
    {
        return null;
    }
}

Used as follows:

DummyObject obj = new DummyObject();
Console.WriteLine("The text: " + obj.GetType() + " is " + obj);

Will throw a NullReferenceException. Turns out the multiple additions are compiled by the C# compiler to a call to String.Concat(object[]). Prior to .NET 4, there is a bug in just that overload of Concat where the object is checked for null, but not the result of ToString():

object obj2 = args[i];
string text = (obj2 != null) ? obj2.ToString() : string.Empty;
// if obj2 is non-null, but obj2.ToString() returns null, then text==null
int length = text.Length;

This is a bug by ECMA-334 §14.7.4:

The binary + operator performs string concatenation when one or both operands are of type string. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string operand is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.