从Linq表达式获取参数值

Question

我有以下课程

public class MyClass
{
    public bool Delete(Product product)
    {
        // some code.
    }
}

现在我有一个看起来像这样的助手类

public class Helper<T, TResult>
{

    public Type Type;
    public string Method;
    public Type[] ArgTypes;
    public object[] ArgValues;

    public Helper(Expression<Func<T, TResult>> expression)
    {
        var body = (System.Linq.Expressions.MethodCallExpression)expression.Body;

        this.Type = typeof(T);
        this.Method = body.Method.Name;
        this.ArgTypes = body.Arguments.Select(x => x.Type).ToArray();
        this.ArgValues = ???
    }
}

这个想法是从某个地方使用这个代码:

// I am returning a helper somewhere
public Helper<T> GetMethod<T>()
{
    var product = GetProduct(1);
    return new Helper<MyClass>(x => x.Delete(product));
}

// some other class decides, when to execute the helper 
// Invoker already exists and is responsible for executing the method
// that is the main reason I don't just comile and execute my Expression
public bool ExecuteMethod<T>(Helper<T> helper)
{
    var instance = new MyClass();
    var Invoker = new Invoker(helper.Type, helper.Method, helper.ArgTypes, helper.ArgValues);
    return (bool)Invoker.Invoke(instance);
}

我被困的地方是如何从表达式本身中提取参数.

我找到了这种方式

((ConstantExpression)((MemberExpression)body.Arguments[0]).Expression).Value

这似乎是一个带有"产品"字段的对象类型,但我相信必须有一个更简单的解决方案.

有什么建议.

更新

为了澄清,我根据我想要的内容修改了我的代码.在我的真正的单词应用程序中,我已经有一个类,但没有表达式树:

var helper = new Helper(typeof(MyClass), "Delete", 
    new Type[] { typeof(Product) }, new object[] {product}));

我的主要原因Helper<T>是如果方法签名有效,则进行编译时检查.

更新2

这是我目前的实现,是否有更好的方法来访问值,而不使用反射？

public Helper(Expression<Func<T, TResult>> expression)
{
    var body = (System.Linq.Expressions.MethodCallExpression)expression.Body;

    this.Type = typeof(T);
    this.Method = body.Method.Name;
    this.ArgTypes = body.Arguments.Select(x => x.Type).ToArray();

    var values = new List<object>();
    foreach(var arg in body.Arguments)
    {
        values.Add(
            (((ConstantExpression)exp.Expression).Value).GetType()
                .GetField(exp.Member.Name)
                .GetValue(((ConstantExpression)exp.Expression).Value);
        );
    }
    this.ArgValues = values.ToArray();
}

Answer 1

这种方法效果很好.它返回Expression>的参数类型和值

    private static KeyValuePair<Type, object>[] ResolveArgs<T>(Expression<Func<T, object>> expression)
    {
        var body = (System.Linq.Expressions.MethodCallExpression)expression.Body;
        var values = new List<KeyValuePair<Type, object>>();

        foreach (var argument in body.Arguments)
        {
            var exp = ResolveMemberExpression(argument);
            var type = argument.Type;

            var value = GetValue(exp);

            values.Add(new KeyValuePair<Type, object>(type, value));
        }

        return values.ToArray();
    }

    public static MemberExpression ResolveMemberExpression(Expression expression)
    {

        if (expression is MemberExpression)
        {
            return (MemberExpression)expression;
        }
        else if (expression is UnaryExpression)
        {
            // if casting is involved, Expression is not x => x.FieldName but x => Convert(x.Fieldname)
            return (MemberExpression)((UnaryExpression)expression).Operand;
        }
        else
        {
            throw new NotSupportedException(expression.ToString());
        }
    }

    private static object GetValue(MemberExpression exp)
    {
        // expression is ConstantExpression or FieldExpression
        if (exp.Expression is ConstantExpression)
        {
            return (((ConstantExpression)exp.Expression).Value)
                    .GetType()
                    .GetField(exp.Member.Name)
                    .GetValue(((ConstantExpression)exp.Expression).Value);    
        }
        else if (exp.Expression is MemberExpression)
        {
            return GetValue((MemberExpression)exp.Expression);
        }
        else
        {
            throw new NotImplementedException();
        }
    }

Answer 2

您可以编译参数表达式,然后调用它来计算值:

var values = new List<object>();
foreach(var arg in body.Arguments)
{
    var value = Expression.Lambda(argument).Compile().DynamicInvoke();
    values.Add(value);
}
this.ArgValues = values.ToArray();