C# Lambda Expressions（對於開發者的運行原理）

Basic Usage

Let's look into an example for Lambda expression where we have a list of integers, and we want to filter out the even numbers.

We can achieve this using the List<T>.FindAll method along with a lambda expression:

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
List<int> evenNumbers = numbers.FindAll(x => x % 2 == 0);

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
List<int> evenNumbers = numbers.FindAll(x => x % 2 == 0);

Dim numbers As New List(Of Integer) From {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Dim evenNumbers As List(Of Integer) = numbers.FindAll(Function(x) x Mod 2 = 0)

In the above example, x => x % 2 == 0 is a lambda expression that takes an integer x as an input parameter and returns true if x is even, and false otherwise. The lambda body expression is executed on each list element.

Use Cases

Using a Lambda expression in modern application programming finds a variety of use cases, including:

1. LINQ Queries: Lambda expressions are extensively used in LINQ (Language Integrated Query) to perform filtering, projection, sorting, and grouping operations on data collections.
2. Event Handling: When working with events and delegates, lambda expressions provide a concise way to define event handlers inline.
3. Asynchronous Programming: In asynchronous programming patterns like Task.Run, lambda expressions can be used to define the asynchronous operation to be executed.
4. Functional Programming: Lambda expressions support functional programming paradigms, enabling operations like map, filter, and reduce on collections.

Expression Lambdas

Lambda expressions with => are called expression lambdas. They take the format:

x => x * x // expression to square a number

x => x * x // expression to square a number

'INSTANT VB TODO TASK: The following line uses invalid syntax:
'x => x * x ' expression to square a number

In the above example, we are creating a square of a number. In expression lambdas, the body can include method calls. However, when generating expression trees intended for evaluation outside the .NET Common Language Runtime (CLR), such as in SQL Server, it is advisable to avoid any method call within lambda expressions.

This is because methods may lack meaning outside the CLR context. Therefore, it's crucial to consider the target environment when constructing an expression tree to ensure compatibility and meaningful interpretation.

Statement Lambdas

Statement lambda expressions allow multiple statements and provide more complex operations:

Func<int, int> mySquareDelegate = (x) =>
{
    return x * x;
};

Console.WriteLine(mySquareDelegate(4)); // Output: 16

Func<int, int> mySquareDelegate = (x) =>
{
    return x * x;
};

Console.WriteLine(mySquareDelegate(4)); // Output: 16

Dim mySquareDelegate As Func(Of Integer, Integer) = Function(x)
	Return x * x
End Function

Console.WriteLine(mySquareDelegate(4)) ' Output: 16

In this code, a delegate is declared that operates on integers and returns their square. The lambda body can encompass multiple statements inside a block {}.

Advanced Techniques

int factor = 2;
Func<int, int> multiplier = x => x * factor;

int factor = 2;
Func<int, int> multiplier = x => x * factor;

Dim factor As Integer = 2
Dim multiplier As Func(Of Integer, Integer) = Function(x) x * factor

Func<int, int, int> add = (a, b) =>
{
    int result = a + b;
    Console.WriteLine($"The sum of {a} and {b} is {result}");
    return result;
};

Func<int, int, int> add = (a, b) =>
{
    int result = a + b;
    Console.WriteLine($"The sum of {a} and {b} is {result}");
    return result;
};

Dim add As Func(Of Integer, Integer, Integer) = Function(a, b)
	Dim result As Integer = a + b
	Console.WriteLine($"The sum of {a} and {b} is {result}")
	Return result
End Function

輸出

結論

C# lambda expressions offer a concise and expressive way to define inline functions, making code more readable, maintainable, and efficient. They find applications in various domains, including LINQ queries, event handling, asynchronous programming, and functional programming.

By mastering lambda expressions, developers can unlock new dimensions of productivity and elegance in their C# codebases.

Whether you're a seasoned C# developer or just starting your journey, understanding and harnessing the power of lambda expressions will undoubtedly elevate your programming skills to new heights.

So dive in, experiment, and embrace the beauty of lambda expressions in your C# projects!