C# closure

last modified July 5, 2023

C# closure tutorial defines closures and shows how to use them.

A closure is an anonymous delegate which maintains access to free variables outside the definition of the block. It can still refer to the variables after the method has finished executing.

var limit = 0;
Predicate<int> greaterThan = e => e > limit;

We have an anonymous lambda expression that refers to the limit variable, even thought it vas not declared in the block or passed as a parameter.

C# closure example

The following is a simple closure example.

var c = CreateCounter();

Console.WriteLine(c());
Console.WriteLine(c());
Console.WriteLine(c());
Console.WriteLine(c());
Console.WriteLine(c());

Func<int> CreateCounter()
{
    int c = 0;
    return () => c = c + 1;
}

The CreateCounter is called several times; after each call, the state (value of c) is retained.

$ dotnet run
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C# closure example II

Closures are often used in LINQ code.

var vals = new int[] {-1, -2, 0, 1, 5, 3};

var limit = 0;
Func<int, bool> greaterThan = e => e > limit;

var res = vals.Where(greaterThan);

foreach (var e in res)
{
    Console.WriteLine(e);
}

In the example, we define a filter delegate. The limit variable that is used in the predicate is a free variable defined outside of the predicate definition.

$ dotnet run 
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C# closure example III

We use a closure in the iterator example.

var words = new List<string> { "sky", "cloud", "rock", "war", "web" };
var it = CreateIterator(words);

string e;

while ((e = it()) != null)
{
    Console.WriteLine(e);
}

Iterator<T> CreateIterator<T>(IList<T> data) where T : class
{
    var i = 0;
    return delegate { return (i < data.Count) ? data[i++] : null; };
}

public delegate T Iterator<T>() where T : class;

We have a generic iterator. In order for the iterator to work, we must retain the state of the increment variable.

Source

What are closures in .NET?

In this article we have worked with closures in C#.

Author

My name is Jan Bodnar and I am a passionate programmer with many years of programming experience. I have been writing programming articles since 2007. So far, I have written over 1400 articles and 8 e-books. I have over eight years of experience in teaching programming.

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