Assigning property of anonymous type via anonymous method

Question

I am new in the functional side of C#, sorry if the question is lame.

Given the following WRONG code:

var jobSummaries = from job in jobs
                   where ...
                   select new 
                   {
                        ID = job.ID,
                        Description = job.Description,
                        FileName = (job) => {
                                  // primitive logic not 
                                  // worth to become a named method
                                  try { return job.Files[0].LocalName); }
                                  catch { return null as string; }
                                 }
                   };

This code produces the following justified compiler error:

cannot assign lambda expression to anonymous type property

The code above would set the delegate to the FileName property. But that is not my aim. I want the code work like this but without naming the method:

var jobSummaries = from job in jobs
                   where ...
                   select new 
                   {
                        ID = job.ID,
                        Description = job.Description,
                        FileName = this.ExtractFileName(job)
                   };

...
private string ExtractFileName(Job job)
{
     try { return Path.GetFileName(job.Files[0].LocalName); }
     catch { return null as string; }
}

Any suggestions?

Answer 1

The call an anonymous function directly, this works:

int result = new Func<int, int>( (int i) =>{ return i + 5; } ).Invoke(3);
// result = 8

But I agree, int result = (i => i + 5)(3); would be way coler =)

Answer 2

As far as I know, you can't inline lambda expressions like that because a lamda expression is an instance itself (of the type Expression<Func<T>> or similar).

However, you can do this (updated with calculation of fileName, since this is now provided by the OP):

var jobSummaries = from job in jobs
                   where ...
                   let fileName = job.Files.Select(f => f.LocalName).FirstOrDefault()
                   select new 
                   {
                        ID = job.ID,
                        Description = job.Description,
                        FileName = fileName
                   };

Notice the use of the let keyword, that lets you extract the filename from the job variable directly inside the LINQ expression.

Answer 3

The compiler is complaining because you are not calling your lambda function, you are defining it. If the compiler would let you, you'd have a FileName property that is a function rather than a value.

If you can write your "primitive logic" as an expression, you can write that directly in the assignment statement.

Answer 4

The MSDN page for CS0828 does indeed say

An anonymous type cannot be initialized with ... a method group or anonymous function.

Now, I'm not qualified to comment on why this should be, but I do noe eit but goes on to suggest

... add a type declaration to the left side of the assignment ...

and this should work for you:

select new 
{
    ID = job.ID,
    Description = job.Description,
    FileName = (Func<Job, string> ((job) => {
                              // some primitive logic extracting 
                              // file name from job object that is not 
                              // worth to become a named method
                        })) (job)
};

Explicitly tell the compiler the type of the lambda, and it will be happy.