Can I declare / use some variable in LINQ? Or can I write following LINQ clearer?

Question

Can I declare / use some variable in LINQ?

For example, can I write following LINQ clearer?

var q = from PropertyDescriptor t in TypeDescriptor.GetPr         


        

Answer 1

You need let:

var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
        let name = t.ComponentType.GetProperty(t.Name)
        where (name != null)
        select name;

If you wanted to do it in query syntax, you could do it in a more efficient (afaik) and cleaner way:

var q = TypeDescriptor
            .GetProperties(instance)
            .Select(t => t.ComponentType.GetProperty(t.Name))
            .Where(name => name != null);

Answer 2

var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
        let u = t.ComponentType.GetProperty(t.Name)
        where (u != null)
        select u;

Answer 3

Yes, using the let keyword:

var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
    let nameProperty = t.ComponentType.GetProperty(t.Name)
    where (nameProperty != null)
    select nameProperty;

Answer 4

There is an alternative that few people know about ( select a into b):

var q = from PropertyDescriptor t in TypeDescriptor.GetProperties(instance)
        select t.ComponentType.GetProperty(t.Name) into u
        where u != null
        select u;

This translates into:

var q = TypeDescriptor.GetProperties(instance)
        .Select(t => t.ComponentType.GetProperty(t.Name))
        .Where(prop => prop != null);

Whereas the let-based version translates to:

var q = TypeDescriptor.GetProperties(instance)
        .Select(t => new { t, prop = t.ComponentType.GetProperty(t.Name) })
        .Where(x => x.prop != null)
        .Select(x => x.prop);

An unnecessary allocation per item because t is still in scope (yet unused). The C# compiler should just optimize that away, but it doesn't (or the language spec does not allow it, not sure).