Using an expression lambda on a generic method on a generic collection

I am trying to understand the below use of a lambda expression. This code is taken from Josh Smith's excellent MVVM demo code (http://msdn.microsoft.com/en-us/magazine/dd419663.aspx#id0090055).

A method is called as follows:

AllCustomersViewModel workspace =
                this.Workspaces.FirstOrDefault(vm => vm is AllCustomersViewModel)
                as AllCustomersViewModel;

As used here, FirstOrDefault has the following definition, as identified by Visual Studio 2010:

public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);

It is not clear to me

1. How does vm get its type? It is not defined elsewhere in the object instance.

2. 开发者_StackOverflow中文版

   How does FirstOrDefault(vm => vm is AllCustomersViewModel) satisfy the source parameter requirement of FirstOrDefault? Is this somehow being implied?

I have been trying to use these resources to parse this out:

http://msdn.microsoft.com/en-us/library/bb397687.aspx

http://msdn.microsoft.com/en-us/library/bb397951.aspx

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1. vm gets its type because Workspaces is a collection that contains a specific type. vm is automatically inferred to be that type.

2. The source parameter of FirstOrDefault is Workspaces. It's an extension method on IEnumerable<T>, so the instance you call it on takes the place of the first parameter. That's what the this in the method signature means.

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Others have answered the question itself. Just as an aside though, this code would be clearer as:

AllCustomersViewModel workspace = this.Workspaces.OfType<AllCustomersViewModel>()
                                                 .FirstOrDefault();

Why bother creating your own operator when LINQ already includes one? (OfType)

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The <TSource, bool> predicate has the first parameter inferred as mentioned above, the second parameter (the boolean) is then supplying by the lambda expression vm => vm is AllCustomersViewModel

The meaning is give me the first (or the default value if none exists) Workspace where the item is an instance of AllCustomersViewModel.