为什么要使用表达式<Func<T>>而不是 Func T ?

Conceptually, Expression<Func<T>> is completely different from Func<T>. Func<T> denotes a delegate which is pretty much a pointer to a method and Expression<Func<T>> denotes a tree data structure for a lambda expression. This tree structure describes what a lambda expression does rather than doing the actual thing. It basically holds data about the composition of expressions, variables, method calls, ... (for example it holds information such as this lambda is some constant + some parameter). You can use this description to convert it to an actual method (with Expression.Compile) or do other stuff (like the LINQ to SQL example) with it. The act of treating lambdas as anonymous methods and expression trees is purely a compile time thing.

Func<int> myFunc = () => 10; // similar to: int myAnonMethod() { return 10; }

将有效地编译成一个什么都不得到并返回 10 的 IL 方法.

will effectively compile to an IL method that gets nothing and returns 10.

Expression<Func<int>> myExpression = () => 10;

将被转换为描述一个没有参数并返回值10的表达式的数据结构:

will be converted to a data structure that describes an expression that gets no parameters and returns the value 10:

虽然它们在编译时看起来相同，但编译器生成的却完全不同.

While they both look the same at compile time, what the compiler generates is totally different.

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