How Do I Ensure Lambda Expressions Capture Variables by Value Instead of by Reference in C#?

Value capture and reference capture in C# Lambda expressions

In C#, Lambda expressions can capture variables in an enclosing scope. By default, these captures are by reference, which means that the lambda expression accesses the same memory location as the original variable. However, in some cases you may want to capture a copy of a variable rather than a reference.

Question:

Consider the following C# code:

<code class="language-csharp">class Program
{
    delegate void Action();
    static void Main(string[] args)
    {
        List<Action> actions = new List<Action>();

        for (int i = 0; i < 10; i++)
            actions.Add(() => Console.WriteLine(i));

        foreach (Action a in actions)
            a();
    }
}</code>

Problem description:

The expected output is for each Lambda expression to print its corresponding i value while iterating through the list. However, in this example, all lambda expressions print the value 10, indicating that they all refer to the same variable i. This is because the lambda expression captures i by reference.

Solution:

To force the lambda expression to capture a copy of i , one solution is to first create a partial copy:

<code class="language-csharp">for (int i = 0; i < 10; i++)
{
    int copy = i; // 创建局部副本
    actions.Add(() => Console.WriteLine(copy));
}</code>

Explanation:

By creating a local copy of i in the loop, the Lambda expression captures the value of copy , which is a copy of the original i at that point in time. This ensures that each Lambda expression has its own copy of the value and the corresponding i value will be printed.

Difference:

The difference between capturing variables by reference and capturing them by value is how they interact with variables outside the lambda expression.

• Capture by reference: Lambda expressions directly access the memory location of the original variable. Any changes made to variables outside the lambda expression will be reflected in the lambda expression and vice versa.
• Capture by value: Lambda expressions access a copy of the original variable value when capturing. Any changes made to variables outside the lambda expression do not affect the copy captured by the lambda expression, and the value of the lambda expression remains unchanged.

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