Why Does My C# Loop Capture the Wrong Variable Value?

Cover variable in the cycle: a strange problem

When using the C#cycle, you may encounter a special problem to capture variables. Let's consider the following scenes:

In contrast to expected, this code prints five "10" instead of the expected sequence 0, 2, 4, 6, 8.

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

int variable = 0;
while (variable < 5)
{
    actions.Add(() => Console.WriteLine(variable * 2));
    ++variable;
}

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

Question: The capture variable in the cycle

All Lambda functions created in the cycle capture the same variable . Therefore, when these functions are called outside, they all quote the final value of

.

variable Solution: Copy rescue variable

In order to solve this problem, you need to create a copy of the variable in the cycle, and then capture it in each lambda function:

By creating a copy, each closure created in the cycle will capture its own

unique value, thereby generating expected output.

<code class="language-csharp">while (variable < 5)
{
    int copy = variable;
    actions.Add(() => Console.WriteLine(copy * 2));
    ++variable;
}</code>

Precautions

variable

Please note that this technology is also applicable to the for and foreach cycle, which references a single variable in multiple iterations. To avoid this problem, it is recommended to obey the C# 5 compiler's processing method of the Foreach cycle, which ensures that each iteration has its own capture variable.

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