LambdaExpression constructor

Question

I saw code like below. My questions are:

1> ()=>Name what does this mean?

2> Is Expression<Func<>> the same as Expression<TDelegate>? How is ()=>Name cast to Expression<Func<>> and which constructor is used? Most Expression classes don't have public constructors. How does C# compiler convert from Lambda to Expression?

3> What is the performance cost of the Parse function?

public class Test
{
    public string Name {get;set;}

    public void Start()
    {
        Parse(()=>Name);
    }

    public string Parse<T>(Expression<Func<T>> exp)
    {
        var mexp = (System.Linq.Expressions.MemberExpression)expression.Body;
        return mexp == null ? "" : mexp.Member.Name;
    }
}

you should measure to answer #3.

Daniel A. White
– Daniel A. White

2015-07-03 22:47:08 +00:00
Commented Jul 3, 2015 at 22:47 — Daniel A. White
– Daniel A. White, Commented Jul 3, 2015 at 22:47

Frank Bryce · Accepted Answer · 2015-07-04 15:45:23Z

5

That is a lambda that takes no arguments, but results in a String.
No. TDelegate is a template argument. Func<T> is a concrete type that satisfies the TDelegate's constraints. The C# compiler will convert the lambda into the appropriate type at compile time.
You should measure in order to answer this question.

edited Jul 4, 2015 at 15:45

Frank Bryce

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answered Jul 3, 2015 at 13:48

Daniel A. White

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3 Comments

Helic Over a year ago

Why it recognize Name property in my class? And in this case, what type the C# complier converts this Lambda to?

Daniel A. White Over a year ago

@Helic that's because it is closed upon.

Helic Over a year ago

so does it mean I can use Parse(()=>{return name;}), but it gives me another exception "A lambda expression with a statement body cannot be converted to an expression tree". Also, please see my edit to the original question. Thanks.

Nitram · Accepted Answer · 2015-07-03 13:51:44Z

3

So ()=>Name is a lambda function. It is basically a function that returns the Name property. In your case this thing is a Func<string> by type.

All that changes a little because you hand it over to a field that is defined as Expression<Func<T>>. In your case you give a Expression<Func<string>>. Basically that makes the lambda function to a expression of a lambda function that gives you not the result of the function but the structure.

Usually a structure like this is used to get the name of a property in a safe way. For example to prevent a obfuscator or a accidental renaming to mess with your code.

answered Jul 3, 2015 at 13:51

Nitram

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2 Comments

Helic Over a year ago

" hand it over " do you mean cast?

Nitram Over a year ago

I have no idea what the C# compiler does at this point. ()=>Name is a Func<string> and it turns to a Expression<Func<string>>. Sadly I can only tell you that it works. I can't tell you why.

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