"if (a() && b != null)" will "a()" always be evaluated?

Solution 1:

Yes, a() will always be evaluated.

Since the condition is evaluated from left to right, a() will always be evaluated, but b != null will only be evaluated if a() returns true.

Here's an exact specification reference for you, from the C# Language Specification version 3.0. My emphases and elisions.

7.11.1 Boolean conditional logical operators

When the operands of && or || are of type bool [...] the operation is processed as follows:

• The operation x && y is evaluated as x ? y : false. In other words, x is first evaluated and converted to type bool. Then, if x is true, y is evaluated and converted to type bool, and this becomes the result of the operation. Otherwise, the result of the operation is false.

Solution 2:

Yes, expressions are evaluated from left to right; so a() will always be called.

See the C# language spec (ECMA 334, paragraph 8.5):

Except for the assignment operators, all binary operators are left-associative, meaning that operations are performed from left to right. For example, x + y + z is evaluated as (x + y) + z.

Solution 3:

The condition is evaluated from left to right. So a() is always executed, but b might not be evaluated depending on the result from a().

Solution 4:

a() will always be evaluated. b != null will only be evaluated if a() evaluates to true.

This is known as short circuit evaluation.