How Do I Prevent Unexpected Behavior When Capturing Variables in C# Lambda Expressions?

Understanding C# Lambda Expression Variable Capture

C# lambda expressions offer a compact way to define anonymous functions. However, their interaction with variables in the surrounding scope can be tricky.

The Problem: Shared Variable Modification

Let's examine a common issue. The following code aims to create a list of actions, each printing a different counter value:

List<Action> actions = new List<Action>();

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

foreach (Action a in actions)
    a();

Surprisingly, this prints "10" ten times. The lambda captures a reference to i, not its value. As the loop modifies i, all lambdas see the final value.

The Solution: Capturing by Value

C# doesn't directly support capture-by-value syntax like C 0x's [=]. The workaround is to create a local copy within the loop's scope:

for (int i = 0; i < 10; i++)
{
    int copy = i; // Create a local copy
    actions.Add(() => Console.WriteLine(copy));
}

Now, each lambda captures a separate copy variable, preserving the correct value for each iteration.

Why Local Copies Matter

The difference lies in variable scope. A reference shares the same memory location; changes are immediately reflected. A copy creates an independent snapshot of the variable's value at the time of capture. Subsequent changes to the original variable are irrelevant to the lambda's captured copy.

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