Why did I get an exception "Cannot implicitly convert type 'bool' to 'long?'" when using the LINQ Sum method and how to fix it?

Question

I have the following code which is working:

IEnumerable<Decimal?> values = getValues();

var sum = values.Where(x => x > 0).Sum();

But if I try:

var sum = values.Sum(x => x > 0);

I get the error:

Cannot implicitly convert type 'bool' to 'long?'

Shouldn't this work either applying the filter in Sum or Where?

Answer 1

Indeed, Sum requires numbers values and not boolean evaluation results.

You need first to filter using Where or Select (not relevant here) then Sum:

var sum = values.Where(x => x != null && x > 0).Sum();

I added the null check because the collection is type of decimal?.

Else you need to write that but this is less speed optimized:

var sum = values.Sum(x => x != null && x > 0 ? x : 0);

Using a selector for Sum is for example usefull when having classes, structs or tuples:

var sum = controls.Sum(control => control.Width);