Why ‘&&’ and not ‘&’?

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In most cases, && and || are preferred over & and | because the former are short-circuited, meaning that the evaluation is canceled as soon as the result is clear.

Example:

if(CanExecute() && CanSave())
{
}

If CanExecute returns false, the complete expression will be false, regardless of the return value of CanSave. Because of this, CanSave is not executed.

This is very handy in the following circumstance:

string value;
if(dict.TryGetValue(key, out value) && value.Contains("test"))
{
    // Do Something
}

TryGetValue returns false if the supplied key is not found in the dictionary. Because of the short-circuiting nature of &&, value.Contains("test") is only executed, when TryGetValue returns true and thus value is not null. If you would use the bitwise AND operator & instead, you would get a NullReferenceException if the key is not found in the dictionary, because the second part of the expression is executed in any case.

A similar but simpler example of this is the following code (as mentioned by TJHeuvel):

if(op != null && op.CanExecute())
{
    // Do Something
}

CanExecute is only executed if op is not null. If op is null, the first part of the expression (op != null) evaluates to false and the evaluation of the rest (op.CanExecute()) is skipped.

Apart from this, technically, they are different, too:
&& and || can only be used on bool whereas & and | can be used on any integral type (bool, int, long, sbyte, …), because they are bitwise operators. & is the bitwise AND operator and | is the bitwise OR operator.

To be very exact, in C#, those operators (&, | [and ^]) are called “Logical operators” (see the C# spec, chapter 7.11). There are several implementations of these operators:

  1. For integers (int, uint, long and ulong, chapter 7.11.1):
    They are implemented to compute the bitwise result of the operands and the operator, i.e. & is implement to compute the bitwise logical AND etc.
  2. For enumerations (chapter 7.11.2):
    They are implemented to perform the logical operation of the underlying type of the enumeration.
  3. For bools and nullable bools (chapter 7.11.3 and 7.11.4):
    The result is not computed using bitwise calculations. The result is basically looked up based on the values of the two operands, because the number of possibilities is so small.
    Because both values are used for the lookup, this implementation isn’t short-circuiting.

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