更新:根据konrad.kruczynski的回答，以下情况适用于。net 4.0及以下版本。

你可以使用MethodImplAttribute类来防止一个方法被内联…

[MethodImpl(MethodImplOptions.NoInlining)]
void SomeMethod()
{
    // ...
}

.．.但是没有办法做相反的事情，强迫它内联。

更新:根据konrad.kruczynski的回答，以下情况适用于。net 4.0及以下版本。

你可以使用MethodImplAttribute类来防止一个方法被内联…

[MethodImpl(MethodImplOptions.NoInlining)]
void SomeMethod()
{
    // ...
}

.．.但是没有办法做相反的事情，强迫它内联。

Lambda表达式是内联函数!我认为，c#没有额外的属性，比如内联或类似的东西!

如果您的程序集将被ngen-ed，您可能想要查看TargetedPatchingOptOut。这将帮助ngen决定是否内联方法。MSDN参考

这仍然只是一个声明性的优化提示，而不是命令式命令。

Cody说得对，但我想提供一个内联函数是什么的例子。

假设你有这样的代码:

private void OutputItem(string x)
{
    Console.WriteLine(x);

    //maybe encapsulate additional logic to decide 
    // whether to also write the message to Trace or a log file
}

public IList<string> BuildListAndOutput(IEnumerable<string> x)
{  // let's pretend IEnumerable<T>.ToList() doesn't exist for the moment
    IList<string> result = new List<string>();

    foreach(string y in x)
    {
        result.Add(y);
        OutputItem(y);
    }
    return result;
}

编译器即时优化器可以选择修改代码，以避免在堆栈上重复调用OutputItem()，这样就好像你写的代码是这样的:

public IList<string> BuildListAndOutput(IEnumerable<string> x)
{
    IList<string> result = new List<string>();

    foreach(string y in x)
    {
        result.Add(y);

        // full OutputItem() implementation is placed here
        Console.WriteLine(y);   
    }

    return result;
}

在本例中，我们可以说OutputItem()函数是内联的。注意，即使从其他地方也调用OutputItem()，它也可能这样做。

经过编辑以显示更可能内联的场景。

The statement "its best to leave these things alone and let the compiler do the work.." (Cody Brocious) is complete rubish. I have been programming high performance game code for 20 years, and I have yet to come across a compiler that is 'smart enough' to know which code should be inlined (functions) or not. It would be useful to have a "inline" statement in c#, truth is that the compiler just doesnt have all the information it needs to determine which function should be always inlined or not without the "inline" hint. Sure if the function is small (accessor) then it might be automatically inlined, but what if it is a few lines of code? Nonesense, the compiler has no way of knowing, you cant just leave that up to the compiler for optimized code (beyond algorithims).