Why Do C# Lambda Expressions in Loops Capture the Final Variable Value, and How Can This Be Fixed?

Understanding C# Lambda Expression Variable Capture in Loops

A frequent problem in C# programming involves lambda expressions within loops. The way lambda expressions capture variables can lead to unexpected results. Let's explore this issue and its solution.

Consider this code:

<code class="language-csharp">List<Func<int>> actions = new List<Func<int>>();

int variable = 0;
while (variable < 10)
{
    actions.Add(() => variable * 2);
    ++variable;
}

foreach (var act in actions)
{
    Console.WriteLine(act.Invoke());
}</code>

The expected output is a sequence of even numbers (0, 2, 4, 6, 8). However, the actual output is five 10s. This happens because each lambda expression captures a reference to the variable, not its value at the time of its creation. By the time the lambdas are invoked, variable has already reached its final value of 10.

The Solution: Creating a Copy

To correct this, we need to create a copy of the loop variable within the scope of each lambda expression:

<code class="language-csharp">List<Func<int>> actions = new List<Func<int>>();

int variable = 0;
while (variable < 10)
{
    int copy = variable; // Create a copy of the variable
    actions.Add(() => copy * 2); 
    ++variable;
}

foreach (var act in actions)
{
    Console.WriteLine(act.Invoke());
}</code>

Now, each lambda expression captures its own independent copy of variable's value at the time of its creation, producing the expected output.

C# 5 and Foreach Loops

It's important to note that this behavior is less of a concern with foreach loops in C# 5 and later versions. The compiler handles the variable capture differently in foreach to prevent this issue. However, the explicit copy method remains a best practice for clarity and compatibility with older C# versions. Using this approach ensures consistent behavior across all C# versions and loop types.

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