LINQ表達式和這個

Question

我想使用LINQ表達式在使用Expression.CompileToMethod方法的類中生成實例方法。 不允許對這部分代碼使用reflection.emit，並且除了LCG方法之外，決不使用linq ：(我需要訪問該類中的一個字段。如何使用LINQ表達式訪問“ this”？

更新：似乎不支持它。 當我嘗試將不是靜態的methodbuilder傳遞給CompileToMethod時，我得到了參數異常。 有人可以確認linq表達式只能用於創建靜態方法嗎？

AssemblyName asmName = new AssemblyName("foo");
            AssemblyBuilder assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(
                asmName,
                AssemblyBuilderAccess.RunAndCollect);


            ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("foo");
            TypeBuilder typeBuilder = moduleBuilder.DefineType(
                "foo",
                TypeAttributes.Public,
                typeof(object));

            FieldInfo field = typeBuilder.DefineField("FooBar", typeof(string), FieldAttributes.Private);
            ConstructorBuilder constructor = typeBuilder.DefineConstructor(MethodAttributes.Public, CallingConventions.Standard, new[] { typeof(string) });

            // want to use linq expressions for constructor too.
            ILGenerator generator = constructor.GetILGenerator();
            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(OpCodes.Call, typeof(object).GetConstructor(Type.EmptyTypes));
            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(OpCodes.Ldarg_1);
            generator.Emit(OpCodes.Stfld, field); 
            generator.Emit(OpCodes.Ret);

            MethodBuilder method = typeBuilder.DefineMethod("Bar", MethodAttributes.Public | MethodAttributes.Static);

            Expression writeLine = Expression.Call(
                null,
                typeof(Console).GetMethod("WriteLine", BindingFlags.Public | BindingFlags.Static, null, new Type[] { typeof(string), typeof(object) }, null),
                Expression.Constant("The value is {0}"),
                Expression.Constant("no this")); //Expression.MakeMemberAccess("this", field)); // need to access this.

            Expression.Lambda<Action>(
                writeLine).CompileToMethod(method);


            Type myType = typeBuilder.CreateType();
            object instance = Activator.CreateInstance(myType, "FooBar");
            MethodInfo barMethod = myType.GetMethod("Bar", BindingFlags.Static | BindingFlags.Public);
            barMethod.Invoke(instance, null);

更新2：似乎不受支持

private void CompileToMethodInternal(MethodBuilder method, DebugInfoGenerator debugInfoGenerator)
{
  ContractUtils.RequiresNotNull(method, "method");
  **ContractUtils.Requires(method.IsStatic, "method");**

Answer 1

您可以只使用Expression.Constant ：

var thisExpr = Expression.Constant(this);
var fieldExpr = Expression.Field(thisExpr, "fieldName");

例如

public class ExprTest
{
    private readonly int value;
    public ExprTest(int value)
    {
        this.value = value;
    }

    public Func<int> GetValueExpr()
    {
        var fieldExpr = Expression.Field(Expression.Constant(this), "value");
        var lambda = Expression.Lambda<Func<int>>(fieldExpr);
        return lambda.Compile();
    }
}

var e = new ExprTest(5);
int i = e.GetValueExpr()();    //i == 5