表達式樹

記錄表達式樹的學習過程

表達式樹將代碼表示為可以檢測、修改、或執行的一種結構，一種定義代碼的結構。

表達式樹是代碼的完整表示形式：可以看到任何子表達式的值。 可以看到方法和屬性名稱。 可以看到任何常數表達式的值。 還可以將自己轉換為可執行的委托，並執行代碼。

通過表達式樹 API，可創建幾乎任何有效代碼構造的樹。 但不能在表達式樹中創建某些 C# 習慣用語，第一 非同步的async、await，第二是迴圈（迴圈有什麼習慣用語？我凌亂了）。

 表達式樹是不可變的數據結構， 只能換新樹。

訪問表達式樹變數

 public static void Main()
 {            
     Expression<Func<int, int>> addFive = num => num + 5;
     // ExpressionType有Add、AddChecked、And、AndAlso、Divide、Equal、Lambda、Multiply等。
     if (addFive.NodeType== ExpressionType.Lambda)
     {
         var lambdaExp = (LambdaExpression)addFive;
         var parameter = lambdaExp.Parameters.First();
         Console.WriteLine(parameter.Name);
         Console.WriteLine(parameter.Type);

         Console.WriteLine(lambdaExp.Parameters.Last().Name); //跟上面同一個
         Console.WriteLine(lambdaExp.Parameters.Last().Type);                
     }
 }

創建表達式樹

public static void Main()
{           
    // 常量表達式樹
    ConstantExpression one = Expression.Constant(1, typeof(int));
    ConstantExpression two = Expression.Constant(2, typeof(int));
    // 二進位表達式樹
    BinaryExpression addition = Expression.Add(one, two);
}

表達式樹API

ExpressionType:（ConvertChecked 和Convert明明c#寫法一樣，怎麼表達不同節點類型，迷茫... Multiply和MultiplyChecked，迷茫...，NewArrayBounds 多維數組）

//
// 摘要:
//     An addition operation, such as a + b, without overflow checking, for numeric
//     operands.
Add = 0,
//
// 摘要:
//     An addition operation, such as (a + b), with overflow checking, for numeric operands.
AddChecked = 1,
//
// 摘要:
//     A bitwise or logical AND operation, such as (a & b) in C# and (a And b) in Visual
//     Basic.
And = 2,
//
// 摘要:
//     A conditional AND operation that evaluates the second operand only if the first
//     operand evaluates to true. It corresponds to (a && b) in C# and (a AndAlso b)
//     in Visual Basic.
AndAlso = 3,
//
// 摘要:
//     An operation that obtains the length of a one-dimensional array, such as array.Length.
ArrayLength = 4,
//
// 摘要:
//     An indexing operation in a one-dimensional array, such as array[index] in C#
//     or array(index) in Visual Basic.
ArrayIndex = 5,
//
// 摘要:
//     A method call, such as in the obj.sampleMethod() expression.
Call = 6,
//
// 摘要:
//     A node that represents a null coalescing operation, such as (a ?? b) in C# or
//     If(a, b) in Visual Basic.
Coalesce = 7,
//
// 摘要:
//     A conditional operation, such as a > b ? a : b in C# or If(a > b, a, b) in Visual
//     Basic.
Conditional = 8,
//
// 摘要:
//     A constant value.
Constant = 9,
//
// 摘要:
//     A cast or conversion operation, such as (SampleType)obj in C#or CType(obj, SampleType)
//     in Visual Basic. For a numeric conversion, if the converted value is too large
//     for the destination type, no exception is thrown.
Convert = 10,
//
// 摘要:
//     A cast or conversion operation, such as (SampleType)obj in C#or CType(obj, SampleType)
//     in Visual Basic. For a numeric conversion, if the converted value does not fit
//     the destination type, an exception is thrown.
ConvertChecked = 11,

//
// 摘要:
//     A division operation, such as (a / b), for numeric operands.
Divide = 12,
//
// 摘要:
//     A node that represents an equality comparison, such as (a == b) in C# or (a =
//     b) in Visual Basic.
Equal = 13,
//
// 摘要:
//     A bitwise or logical XOR operation, such as (a ^ b) in C# or (a Xor b) in Visual
//     Basic.
ExclusiveOr = 14,
//
// 摘要:
//     A "greater than" comparison, such as (a > b).
GreaterThan = 15,
//
// 摘要:
//     A "greater than or equal to" comparison, such as (a >= b).
GreaterThanOrEqual = 16,
//
// 摘要:
//     An operation that invokes a delegate or lambda expression, such as sampleDelegate.Invoke().
Invoke = 17,
//
// 摘要:
//     A lambda expression, such as a => a + a in C# or Function(a) a + a in Visual
//     Basic.
Lambda = 18,
//
// 摘要:
//     A bitwise left-shift operation, such as (a << b).
LeftShift = 19,
//
// 摘要:
//     A "less than" comparison, such as (a < b).
LessThan = 20,
//
// 摘要:
//     A "less than or equal to" comparison, such as (a <= b).
LessThanOrEqual = 21,
//
// 摘要:
//     An operation that creates a new System.Collections.IEnumerable object and initializes
//     it from a list of elements, such as new List<SampleType>(){ a, b, c } in C# or
//     Dim sampleList = { a, b, c } in Visual Basic.
ListInit = 22,
//
// 摘要:
//     An operation that reads from a field or property, such as obj.SampleProperty.
MemberAccess = 23,
//
// 摘要:
//     An operation that creates a new object and initializes one or more of its members,
//     such as new Point { X = 1, Y = 2 } in C# or New Point With {.X = 1, .Y = 2} in
//     Visual Basic.
MemberInit = 24,

 //
        // 摘要:
        //     An arithmetic remainder operation, such as (a % b) in C# or (a Mod b) in Visual
        //     Basic.
        Modulo = 25,
        //
        // 摘要:
        //     A multiplication operation, such as (a * b), without overflow checking, for numeric
        //     operands.
        Multiply = 26,
        //
        // 摘要:
        //     An multiplication operation, such as (a * b), that has overflow checking, for
        //     numeric operands.
        MultiplyChecked = 27,
        //
        // 摘要:
        //     An arithmetic negation operation, such as (-a). The object a should not be modified
        //     in place.
        Negate = 28,
        //
        // 摘要:
        //     A unary plus operation, such as (+a). The result of a predefined unary plus operation
        //     is the value of the operand, but user-defined implementations might have unusual
        //     results.
        UnaryPlus = 29,
        //
        // 摘要:
        //     An arithmetic negation operation, such as (-a), that has overflow checking. The
        //     object a should not be modified in place.
        NegateChecked = 30,
        //
        // 摘要:
        //     An operation that calls a constructor to create a new object, such as new SampleType().
        New = 31,
        //
        // 摘要:
        //     An operation that creates a new one-dimensional array and initializes it from
        //     a list of elements, such as new SampleType[]{a, b, c} in C# or New SampleType(){a,
        //     b, c} in Visual Basic.
        NewArrayInit = 32,
        //
        // 摘要:
        //     An operation that creates a new array, in which the bounds for each dimension
        //     are specified, such as new SampleType[dim1, dim2] in C# or New SampleType(dim1,
        //     dim2) in Visual Basic.
        NewArrayBounds = 33,
        //
        // 摘要:
        //     A bitwise complement or logical negation operation. In C#, it is equivalent to
        //     (~a) for integral types and to (!a) for Boolean values. In Visual Basic, it is
        //     equivalent to (Not a). The object a should not be modified in place.
        Not = 34,

（Quote節點類型是什麼？）

 //
        // 摘要:
        //     A bitwise complement or logical negation operation. In C#, it is equivalent to
        //     (~a) for integral types and to (!a) for Boolean values. In Visual Basic, it is
        //     equivalent to (Not a). The object a should not be modified in place.
        Not = 34,
        //
        // 摘要:
        //     An inequality comparison, such as (a != b) in C# or (a <> b) in Visual Basic.
        NotEqual = 35,
        //
        // 摘要:
        //     A bitwise or logical OR operation, such as (a | b) in C# or (a Or b) in Visual
        //     Basic.
        Or = 36,
        //
        // 摘要:
        //     A short-circuiting conditional OR operation, such as (a || b) in C# or (a OrElse
        //     b) in Visual Basic.
        OrElse = 37,
        //
        // 摘要:
        //     A reference to a parameter or variable that is defined in the context of the
        //     expression. For more information, see System.Linq.Expressions.ParameterExpression.
        Parameter = 38,
        //
        // 摘要:
        //     A mathematical operation that raises a number to a power, such as (a ^ b) in
        //     Visual Basic.
        Power = 39,
        //
        // 摘要:
        //     An expression that has a constant value of type System.Linq.Expressions.Expression.
        //     A System.Linq.Expressions.ExpressionType.Quote node can contain references to
        //     parameters that are defined in the context of the expression it represents.
        Quote = 40,
        //
        // 摘要:
        //     A bitwise right-shift operation, such as (a >> b).
        RightShift = 41,

 Expression：BinaryExpression、IndexExpression、MethodCallExpression、UnaryExpression、BlockExpression、GotoExpression、DynamicExpression、LambdaExpression、MemberExpression等。

ExpressionVisitor。