Reputation: 14970

Why does this not compile in C#?

Why does this work:

if ("xx".StartsWith("x"))
{

}

But this doesn't:

if ("xx" + "xx".StartsWith("x"))
{

}

Compiler says error CS0029: Cannot implicitly convert type 'string' to 'bool'

Upvotes: 5

Answers (12)

Kyle Trauberman

Reputation: 25694

Wrap it in Parens

if (("xx" + "xx").StartsWith("x"))
{

}

The reason for the error is that a string plus a bool = a string, and the if statement is expecting a bool.

Upvotes: 16

Bobby

Reputation: 11576

Because the Compiler is evaluating function-calls first, so he will try to concatenate "xx" with true:

if("xx" + "xx".StartsWith("x")) {
// becomes
if("xx" + true) {

This implicit conversion is not possible during runtime. You'll need to tell the compiler to concatenate the strings first:

if(("xx" + "xx").StartsWith("x")) {

Upvotes: 5

CodesInChaos

Reputation: 108880

The member access operator . has higher priority than the + operator.

Check C# Operators (MSDN) for the C# operator priorities. In particular it lists x.y as "primary" which is higher than binary + in "additive".

This means "xx" + "xx".StartsWith("x") is interpreted as
"xx" + ("xx".StartsWith("x")) and that doesn't compile. It concats a string and a bool which gives you a string. But you can't use a string as a condition in an if statement since it's not bool (and doesn't implement the true operator either)

It seems you expected it to be interpreted as ("xx" + "xx").StartsWith("x")) where you first concat the strings and then call StartsWith on the combined string.

Upvotes: 25

sourc3

Reputation: 76

It has to do with the precedence of operators. In your case, StartsWith returns bool and when combined with a string addition string + bool does not return a bool, it returns a string while the if ( ) expects a bool.

You can change the precedence of operations by using parenthesis.

Upvotes: 5

d-live

Reputation: 8036

if () expects a boolean value which your expression isnt. This is in contrast from C/C++ which expects an int value as a condition.

Upvotes: 2

slandau

Reputation: 24102

"xx".StartsWith("x")

is returning a bool and when you try and add "xx" to it, it's converting it to a string.

Change your code to

if (("xx" + "xx").StartsWith("x"))
{

}

Upvotes: 2

Chad

Reputation: 19619

The reason is because the String.StartsWith() method returns a boolean (ref). So you are applying the + operator on a string and boolean, which doesn't work. If you wanted to concate the strings, then call String.StartsWith() on the newly created one; add some parenthesis:

if(("xx" + "xx").StartsWith("x"))
{

}

Hope this helps

Upvotes: 0

Damian Leszczyński - Vash

Reputation: 30875

Because in the if statement a logical expression is expected not string.

Upvotes: 2

Greg Buehler

Reputation: 3894

"xx".StartsWith("x") returns a boolean value, but "xx" + "xx".StartsWith("x") returns a string that would be either "xxTrue" or "xxFalse".

Upvotes: 2

Chad

Reputation: 255

The concatenation operator (+) is doing an implicit cast to a string in the second example. That example is also checking to see if a string is set not if it starts with something.

Upvotes: 0

Kevin Hsu

Reputation: 1756

"xx" + "xx".StartsWith("x") is not a boolean expression. It evaluates to "xxTrue".

Upvotes: 0

Antony Blazer

Reputation: 705

Because in the second case you try to compile such code:

 if ("xx" + true)
 {
 }

Upvotes: 18

Why does this not compile in C#?

Answers (12)

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