Why does a Lambda Expression inside a Foreach Loop capture the last value of the loop variable?-C++-php.cn

Why does a Lambda Expression inside a Foreach Loop capture the last value of the loop variable?

Mary-Kate Olsen

Release： 2024-11-01 00:01:28

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Why does a Lambda Expression inside a Foreach Loop capture the last value of the loop variable?

Failed Lambda Expression in Foreach Loop

Consider the following code that initializes a list of delegates, each of which calls a method SayGreetingToType with a specific type and greeting:

<code class="csharp">public class MyClass
{
    public delegate string PrintHelloType(string greeting);

    public void Execute()
    {
        Type[] types = new Type[] { typeof(string), typeof(float), typeof(int) };
        List<PrintHelloType> helloMethods = new List<PrintHelloType>();

        foreach (var type in types)
        {
            // Initialize the lambda expression with the captured variable 'type'
            var sayHello = new PrintHelloType(greeting => SayGreetingToType(type, greeting));
            helloMethods.Add(sayHello);
        }

        // Call the delegates with the greeting "Hi"
        foreach (var helloMethod in helloMethods)
        {
            Console.WriteLine(helloMethod("Hi"));
        }
    }

    public string SayGreetingToType(Type type, string greetingText)
    {
        return greetingText + " " + type.Name;
    }
}</code>

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Upon executing this code, you would expect to see:

Hi String
Hi Single
Hi Int32

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However, due to closure behavior, the last type in the types array, Int32, is captured by all lambda expressions. As a result, all delegates invoke SayGreetingToType with the same type, leading to the unexpected output of:

Hi Int32
Hi Int32
Hi Int32

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The Solution

To resolve this issue, we need to capture the value of the loop variable within the lambda expression instead of the variable itself:

<code class="csharp">public class MyClass
{
    public delegate string PrintHelloType(string greeting);

    public void Execute()
    {
        Type[] types = new Type[] { typeof(string), typeof(float), typeof(int) };
        List<PrintHelloType> helloMethods = new List<PrintHelloType>();

        foreach (var type in types)
        {
            // Capture a copy of the current 'type' value using a new variable
            var newType = type;

            // Initialize the lambda expression with the new variable 'newType'
            var sayHello = new PrintHelloType(greeting => SayGreetingToType(newType, greeting));
            helloMethods.Add(sayHello);
        }

        // Call the delegates with the greeting "Hi"
        foreach (var helloMethod in helloMethods)
        {
            Console.WriteLine(helloMethod("Hi"));
        }
    }

    public string SayGreetingToType(Type type, string greetingText)
    {
        return greetingText + " " + type.Name;
    }
}</code>

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This modification ensures that each lambda expression has its own copy of the type, allowing it to invoke SayGreetingToType with the correct type argument.

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