.NET 3.5表达式树中的赋值

Question

是否可以将赋值编码到表达式树中？

Answer 1

不，我不这么认为。

当然，C#编译器在转换lambda表达式时不允许它：

int x;
Expression<Func<int,int>> foo = (x=y); // Assign to x and return value

这就产生了错误：

CS0832: An expression tree may not contain an assignment operator

Answer 2

您应该能够使用.NET 4.0库来完成此操作。通过将Microsoft.Scripting.Core.dll导入您的.NET 3.5项目。

我使用的是DLR 0.9-版本1.0中的Expession.Block和Expression.Scope可能会有一些更改（您可以从http://www.codeplex.com/dlr/Thread/View.aspx?ThreadId=43234)

下面的示例将向您展示。

using System;
using System.Collections.Generic;
using Microsoft.Scripting.Ast;
using Microsoft.Linq.Expressions;
using System.Reflection;
namespace dlr_sample
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Expression> statements = new List<Expression>();
            ParameterExpression x = Expression.Variable(typeof(int), "r");
            ParameterExpression y = Expression.Variable(typeof(int), "y");
            statements.Add(
                Expression.Assign(
                    x,
                    Expression.Constant(1)
                )
             );
            statements.Add(
                Expression.Assign(
                    y,
                    x
                )
             );
            MethodInfo cw = typeof(Console).GetMethod("WriteLine", new Type[] { typeof(int) });
            statements.Add(
                Expression.Call(
                    cw,
                    y
                )
            );
            LambdaExpression lambda = Expression.Lambda(Expression.Scope(Expression.Block(statements), x, y));
            lambda.Compile().DynamicInvoke();
            Console.ReadLine();
        }
    }
}

Answer 3

我的扩展方法：

/// <summary>
/// Provides extensions for converting lambda functions into assignment actions
/// </summary>
public static class ExpressionExtenstions
{
    /// <summary>
    /// Converts a field/property retrieve expression into a field/property assign expression
    /// </summary>
    /// <typeparam name="TInstance">The type of the instance.</typeparam>
    /// <typeparam name="TProp">The type of the prop.</typeparam>
    /// <param name="fieldGetter">The field getter.</param>
    /// <returns></returns>
    public static Expression<Action<TInstance, TProp>> ToFieldAssignExpression<TInstance, TProp>
        (
        this Expression<Func<TInstance, TProp>> fieldGetter
        )
    {
        if (fieldGetter == null)
            throw new ArgumentNullException("fieldGetter");
        if (fieldGetter.Parameters.Count != 1 || !(fieldGetter.Body is MemberExpression))
            throw new ArgumentException(
                @"Input expression must be a single parameter field getter, e.g. g => g._fieldToSet  or function(g) g._fieldToSet");
        var parms = new[]
                        {
                            fieldGetter.Parameters[0],
                            Expression.Parameter(typeof (TProp), "value")
                        };
        Expression body = Expression.Call(AssignmentHelper<TProp>.MethodInfoSetValue,
                                          new[] {fieldGetter.Body, parms[1]});
        return Expression.Lambda<Action<TInstance, TProp>>(body, parms);
    }

    public static Action<TInstance, TProp> ToFieldAssignment<TInstance, TProp>
        (
        this Expression<Func<TInstance, TProp>> fieldGetter
        )
    {
        return fieldGetter.ToFieldAssignExpression().Compile();
    }
    #region Nested type: AssignmentHelper
    private class AssignmentHelper<T>
    {
        internal static readonly MethodInfo MethodInfoSetValue =
            typeof (AssignmentHelper<T>).GetMethod("SetValue", BindingFlags.NonPublic | BindingFlags.Static);
        private static void SetValue(ref T target, T value)
        {
            target = value;
        }
    }
    #endregion
}

Answer 4

正如Jon Skeet和TraumaPony已经说过的，Expression.Assign在.NET 4之前是不可用的。以下是如何解决这一缺失的另一个具体例子：

public static class AssignmentExpression
{
    public static Expression Create(Expression left, Expression right)
    {
        return
            Expression.Call(
               null,
               typeof(AssignmentExpression)
                  .GetMethod("AssignTo", BindingFlags.NonPublic | BindingFlags.Static)
                  .MakeGenericMethod(left.Type),
               left,
               right);
    }
    private static void AssignTo<T>(ref T left, T right)  // note the 'ref', which is
    {                                                     // important when assigning
        left = right;                                     // to value types!
    }
}

然后简单地调用AssignmentExpression.Create()来代替Expression.Assign()。

Answer 5

您可能可以通过连接表达式树来解决它。调用lambda函数，其中参数是受让人的值。