C# & VB.NET Conversion Pocket Reference

Syntax Differences

|

35

Short-Circuiting

In addition to syntax differences, if statements in both lan-

guages differ in semantics. The main difference is the use of

short-circuiting in C# by default. Consider the following

example:

static bool DoTask1( )

{

return true;

}

static bool DoTask2( )

{

return false;

}

static void Main(string[] args)

{

//it is unnecessary to execute

//DoTask2 because DoTask1 returns

//True, which is enough to satisfy

//the or clause

if (DoTask1() || DoTask2( ))

{

}

//it is unnecessary to execute

//DoTask1 because DoTask2 returns

//False, which means the clause is

//False

if (DoTask2() && DoTask1( ))

{

}

}

In this example, the function DoTask1 always returns true,

and the function DoTask2 always returns false. C# uses a

technique called short-circuiting, which means that if part of

the statement makes the entire statement true or false, there

is no need to process the rest of the statement. For example,

in an

or operation, if at least one of the clauses is true, then

the entire statement is true. In the example above, DoTask1

returns true; therefore, there is no need to execute DoTask2.

C#

36

|

C# & VB.NET Conversion Pocket Reference

The second half of the code example contains an and opera-

tion. In an

and operation, all clauses have to be true. If one of

the clauses is false, there is no need to evaluate the rest of the

statement. In this example, DoTask2 returns false; therefore,

there is no need to evaluate DoTask1.

VB does not short-circuit by default. In VB, all parts of the

conditional statement are executed unless you use the spe-

cial keywords

AndAlso or OrElse. For example:

Shared Function DoTask1( ) As Boolean

Return True

End Function

Shared Function DoTask2( ) As Boolean

Return False

End Function

Shared Sub Main( )

'VB evaluates each portion of the

'if clause without short-circuiting

If DoTask1() Or DoTask2( )

End If

If DoTask2() And DoTask1( )

End If

End Sub

Both the Or clause and the And clause will execute DoTask1

and DoTask2 without short-circuiting. If you would like to

short-circuit, you must replace

And with the AndAlso key-

word and

Or with the OrElse keyword, as follows:

Shared Function DoTask1( ) As Boolean

Return True

End Function

Shared Function DoTask2( ) As Boolean

Return False

End Function

Shared Sub Main( )

If DoTask1() OrElse DoTask2( )

End If

VB

VB