使用表達式樹為新類創建Lambda表達式選擇器

解決這個問題的方法是編寫等效代碼，然後對其進行反編譯。例如：

using System;
using System.Linq.Expressions;

class Program
{
    static void Main()
    {
        ShowMeTheLambda<Foo, Allocation>(s => new Allocation
        {
            Id = s.Id,
            UnitName = s.UnitName,
            Address = s.NewAddress,
            Tel = s.NewTel
        });
    }
    static void ShowMeTheLambda<TFrom, TTo>(Expression<Func<TFrom, TTo>> lambda)
    { }
}
class Foo
{
    public int Id { get; set; }
    public string UnitName { get; set; }
    public string NewTel { get; set; }
    public string NewAddress { get; set; }
}
class Allocation
{
    public int Id { get; set; }
    public string UnitName { get; set; }
    public string Tel { get; set; }
    public string Address { get; set; }
}

現在，如果我編譯它並用“反射器”反編譯它，我得到：

private static void Main()
{
    ParameterExpression expression;
    MemberBinding[] bindings = new MemberBinding[] { Expression.Bind((MethodInfo) methodof(Allocation.set_Id), Expression.Property(expression = Expression.Parameter(typeof(Foo), "s"), (MethodInfo) methodof(Foo.get_Id))), Expression.Bind((MethodInfo) methodof(Allocation.set_UnitName), Expression.Property(expression, (MethodInfo) methodof(Foo.get_UnitName))), Expression.Bind((MethodInfo) methodof(Allocation.set_Address), Expression.Property(expression, (MethodInfo) methodof(Foo.get_NewAddress))), Expression.Bind((MethodInfo) methodof(Allocation.set_Tel), Expression.Property(expression, (MethodInfo) methodof(Foo.get_NewTel))) };
    ParameterExpression[] parameters = new ParameterExpression[] { expression };
    ShowMeTheLambda<Foo, Allocation>(Expression.Lambda<Func<Foo, Allocation>>(Expression.MemberInit(Expression.New(typeof(Allocation)), bindings), parameters));

}

注意： memberof和methodof實際上並不存在於C＃中 - 您可以通過反射手動獲取方法信息，也可以使用Expression.PropertyOrField 。因此我們可以將其重寫為：

ParameterExpression expression = Expression.Parameter(typeof(Foo), "s");
MemberBinding[] bindings = new MemberBinding[]
{
    Expression.Bind(typeof(Allocation).GetProperty(nameof(Allocation.Id)), Expression.PropertyOrField(expression, nameof(Foo.Id))),
    Expression.Bind(typeof(Allocation).GetProperty(nameof(Allocation.UnitName)), Expression.PropertyOrField(expression, nameof(Foo.UnitName))),
    Expression.Bind(typeof(Allocation).GetProperty(nameof(Allocation.Address)), Expression.PropertyOrField(expression, nameof(Foo.NewAddress))),
    Expression.Bind(typeof(Allocation).GetProperty(nameof(Allocation.Tel)), Expression.PropertyOrField(expression, nameof(Foo.NewTel))),
};
ParameterExpression[] parameters = new ParameterExpression[] { expression };
var lambda = Expression.Lambda<Func<Foo, Allocation>>(Expression.MemberInit(Expression.New(typeof(Allocation)), bindings), parameters);