yield return 在C# 中用於按需返回序列中的項(xiàng)，提升處理大數(shù)據(jù)或動(dòng)態(tài)序列的效率。其核心機(jī)制是編譯器生成狀態(tài)機(jī)實(shí)現(xiàn)延遲執(zhí)行，每次迭代時(shí)只返回一個(gè)元素，如讀取大文件、查詢數(shù)據(jù)庫(kù)或過濾目錄文件時(shí)可避免一次性加載全部數(shù)據(jù)。使用時(shí)需注意：方法不可含ref 或out 參數(shù)，不可在try-catch 中使用yield return，且需小心循環(huán)內(nèi)昂貴操作的重複執(zhí)行，此外數(shù)據(jù)變動(dòng)可能影響枚舉結(jié)果。

The yield return statement in C# is a powerful but often misunderstood feature used primarily in methods that return sequences — like IEnumerable<t></t> . Instead of building a whole collection and returning it all at once, yield return lets you return items one at a time, on demand. This makes your code more efficient, especially when working with large datasets or infinite sequences.

How Does yield return Work?

When you use yield return , you're essentially telling the compiler to generate a state machine behind the scenes. This machine keeps track of where you are in the sequence so that each item is only generated when it's actually needed — a concept known as deferred execution.

Here's a basic example:

 public IEnumerable<int> GetNumbers()
{
    yield return 1;
    yield return 2;
    yield return 3;
}

When someone iterates over this method using a foreach loop, each number is returned one by one as the loop progresses. The method doesn't run all at once — it pauses after each yield return and resumes when the next item is requested.

This is especially useful for:

• Reading large files line by line
• Querying databases without loading all results at once
• Generating sequences dynamically

When to Use yield return

You'll typically want to use yield return when:

• You're returning a sequence from a method
• You don't want or need to build a full list up front
• You're working with data that can be processed incrementally

For example, imagine you're scanning a directory with thousands of files:

 public IEnumerable<string> GetLargeFileNames(string path)
{
    foreach (var file in Directory.EnumerateFiles(path))
    {
        if (new FileInfo(file).Length > 1_000_000)
        {
            yield return file;
        }
    }
}

This way, you don't load all file names into memory at once, and you only return what's needed when it's needed.

Things to Keep in Mind

Using yield return isn't without caveats. Here are a few important points:

• Methods using yield return can't have ref or out parameters.
• You can't use try-catch blocks around yield return statements (though try-finally works).
• Each iteration re-enters the method, so any expensive logic inside the loop might be called multiple times — be careful with performance.

Also, because of how deferred execution works, if the underlying data changes between the time the method is called and when it's enumerated, the results can vary. This is something to watch for when working with dynamic or shared data.

Summary

yield return is a clean and efficient way to produce sequences in C#. It helps reduce memory usage and improves performance by generating items only when needed. It's ideal for filtering, streaming, or lazy-loading data.

That's basically it — not too complicated, but very useful once you understand how and when to use it.

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