C# – What Does the ‘=>’ Syntax Mean?

c++syntax

I just came over this syntax in some of the questions in this forum, but Google and any other searchengine tends to block out anything but letters and number in the search so it is impossible to search out "=>".

So can anyone tell me what it is and how it is used?

Best Answer

It's the lambda operator.

From C# 3 to C# 5, this was only used for lambda expressions. These are basically a shorter form of the anonymous methods introduced in C# 2, but can also be converted into expression trees.

As an example:

Func<Person, string> nameProjection = p => p.Name;

is equivalent to:

Func<Person, string> nameProjection = delegate (Person p) { return p.Name; };

In both cases you're creating a delegate with a Person parameter, returning that person's name (as a string).

In C# 6 the same syntax is used for expression-bodied members, e.g.

// Expression-bodied property
public int IsValid => name != null && id != -1;

// Expression-bodied method
public int GetHashCode() => id.GetHashCode();

Related Solutions

C# – What Does () => Mean?

Search StackOverflow for "lambda".

Specifically:

() => Console.WriteLine("Hi!");

That means "a method that takes no arguments and returns void, and when you call it, it writes the message to the console."

You can store it in an Action variable:

Action a = () => Console.WriteLine("Hi!");

And then you can call it:

a();

C# – Understanding the => Operator in Properties and Methods

What you're looking at is an expression-bodied member not a lambda expression.

When the compiler encounters an expression-bodied property member, it essentially converts it to a getter like this:

public int MaxHealth
{
    get
    {
        return Memory[Address].IsValid ? Memory[Address].Read<int>(Offs.Life.MaxHp) : 0;
    }
}

(You can verify this for yourself by pumping the code into a tool called TryRoslyn.)

Expression-bodied members - like most C# 6 features - are just syntactic sugar. This means that they don’t provide functionality that couldn't otherwise be achieved through existing features. Instead, these new features allow a more expressive and succinct syntax to be used

As you can see, expression-bodied members have a handful of shortcuts that make property members more compact:

There is no need to use a return statement because the compiler can infer that you want to return the result of the expression
There is no need to create a statement block because the body is only one expression
There is no need to use the get keyword because it is implied by the use of the expression-bodied member syntax.

I have made the final point bold because it is relevant to your actual question, which I will answer now.

The difference between...

// expression-bodied member property
public int MaxHealth => x ? y:z;

And...

// field with field initializer
public int MaxHealth = x ? y:z;

Is the same as the difference between...

public int MaxHealth
{
    get
    {
        return x ? y:z;
    }
}

And...

public int MaxHealth = x ? y:z;

Which - if you understand properties - should be obvious.

Just to be clear, though: the first listing is a property with a getter under the hood that will be called each time you access it. The second listing is is a field with a field initializer, whose expression is only evaluated once, when the type is instantiated.

This difference in syntax is actually quite subtle and can lead to a "gotcha" which is described by Bill Wagner in a post entitled "A C# 6 gotcha: Initialization vs. Expression Bodied Members".

While expression-bodied members are lambda expression-like, they are not lambda expressions. The fundamental difference is that a lambda expression results in either a delegate instance or an expression tree. Expression-bodied members are just a directive to the compiler to generate a property behind the scenes. The similarity (more or less) starts and end with the arrow (=>).

I'll also add that expression-bodied members are not limited to property members. They work on all these members:

Properties
Indexers
Methods
Operators

Added in C# 7.0

However, they do not work on these members:

Nested Types
Events
Fields

Best Answer

Related Solutions

C# – What Does () => Mean?

C# – Understanding the => Operator in Properties and Methods

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