Null-Coalescing Operator and Lambda Expression in C#

.netc++lambdanull-coalescing-operator

take a look at the following code I attempted to write inside a constructor:

private Predicate<string> _isValid;

//...

Predicate<string> isValid = //...;
this._isValid = isValid ?? s => true;

The code doesn't compile – just "invalid expression term"s and so one.

In contrast that does compile and I could just use it:

this._isValid = isValid ?? new Predicate<string>(s => true);

However, I still wonder why this syntax is not allowed.

Any ideas?

Best Answer

this._isValid = isValid ?? (s => true);

Will work :)

It parsed it this way:

this._isValid = (isValid ?? s) => true;

which does not make any sense.

Related Solutions

C# Null Coalescing Operator – Why It May Return Null

This could be a problem with the debugger rather than with the code. Trying printing out the value or doing a null check after the statement with the coalesce operator.

C# Coalesce Operator – Usage and Examples

"the left part is simply ignored"? How likely do you really think that is?

Operator precedence dictates that your first version is parsed like so:

bool? result = (someCondition ?? someConditionOverride) 
                   ? false 
                   : (bool?)null;

someCondition isn't null, and it is true. Therefore, this expression evaluates as true:

(someCondition ?? someConditionOverride)

So we get the ? branch, and the whole expression returns false, just like you told it to.

Adding the brackets that you added completely changes the meaning of the expression. It fixes your confusion, in a limited sense, by bringing the actual meaning of the expression in line with your original intent; but the compiler is never confused. In C#, compiler confuses you.

To reduce my own confusion, I never rely on operator precedence. I parenthesize everything. If I had designed the language, the grammar would require it (unless Eric comes by and tells me why that's actually not such a good idea after all, for some reason that will make perfect sense to me once Eric explains it).

UPDATE: Prediction validated: Eric came by and said that too many people would find it ridiculous to have to parenthesize a + b * c, and if you allow that, there's no reasonable way to define the exceptions. Well, if they won't tolerate being forced to parenthesize everything, they won't, however much I think they should.

Best Answer

Related Solutions

C# Null Coalescing Operator – Why It May Return Null

C# Coalesce Operator – Usage and Examples

Related Question