Question

小时。我正在尝试构建linq查询，动态生成查询以动态发送字段的自定义排序。

我是

的构建逻辑

Expression<Func<string, int>> SpaceStringSortExpression = (a) => a.StartsWith(" ") ? 2 : 1;

此代码（SpaceStringSortExpression.ToString（））的签名是“a =＆gt; IIF（a.StartsWith（\”\“），2,1）”

为了做到这一点，我做了：

ParameterExpression parameter = Expression.Parameter(typeof(TSource), "p1");
            Expression orderByProperty = Expression.Property(parameter, propertyName);
            ConstantExpression c = Expression.Constant(" ", typeof(string));
            MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
            Expression call = Expression.Call(orderByProperty, mi, c);
            Expression<Func<TSource, bool>> lambda = Expression.Lambda<Func<TSource, bool>>(call, parameter);
            ConditionalExpression t = Expression.IfThenElse(call, Expression.Constant(2), Expression.Constant(1));
            //t.tostring() - IIF(p1.Login.StartsWith(" "), 2, 1)
            LambdaExpression callt = Expression.Lambda(t, new[] { parameter });
            //callt.tostring() = p1 => IIF(p1.Login.StartsWith(" "), 2, 1)

但我无法将结果传递给OrderBy

MethodInfo genericMethod;

            genericMethod = OrderByMethod.MakeGenericMethod

            genericMethod = OrderByDescendingMethod.MakeGenericMethod
           (new[] { typeof(TSource), typeof(Int32) });

        object ret = genericMethod.Invoke(null, new object[] { source, callt });
        return (IQueryable<TSource>)ret;

我有（从locolised IIS翻译）

Unable to convert "System.Linq.Expressions.Expression`1[System.Action`1[XXXX.User]]" to type "System.Linq.Expressions.Expression`1[System.Func`2[XXXX.User,System.Int32]]".

整个代码是：

public static IQueryable<TSource> OrderByPropertyRegardingWhiteSpaces<TSource>
        (this IQueryable<TSource> source, string propertyName, bool ascDirection = true)
        {
  ParameterExpression parameter = Expression.Parameter(typeof(TSource), "p1");
            Expression orderByProperty = Expression.Property(parameter, propertyName);
            ConstantExpression c = Expression.Constant(" ", typeof(string));
            MethodInfo mi = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
            Expression call = Expression.Call(orderByProperty, mi, c);
            Expression<Func<TSource, bool>> lambda = Expression.Lambda<Func<TSource, bool>>(call, parameter);
            ConditionalExpression t = Expression.IfThenElse(call, Expression.Constant(2), Expression.Constant(1));
            //t.tostring() - IIF(p1.Login.StartsWith(" "), 2, 1)
            LambdaExpression callt = Expression.Lambda(t, new[] { parameter });
            //callt.tostring() = p1 => IIF(p1.Login.StartsWith(" "), 2, 1)
MethodInfo genericMethod;
genericMethod = OrderByMethod.MakeGenericMethod
                (new[] { typeof(TSource), typeof(Int32) });
object ret = genericMethod.Invoke(null, new object[] { source, callt });
            return (IQueryable<TSource>)ret;
   }



        private static readonly MethodInfo OrderByMethod =
        typeof(Queryable).GetMethods()
            .Where(method => method.Name == "OrderBy")
            .Where(method => method.GetParameters().Length == 2)
            .Single();

有人可以帮我吗？

简单的例子（仅按动态参数排序）工作正常：

public static IQueryable<TSource> OrderByProperty<TSource>
        (this IQueryable<TSource> source, string propertyName, bool ascDirection = true)
        {
            ParameterExpression parameter = Expression.Parameter(typeof(TSource), "p1");
            Expression orderByProperty = Expression.Property(parameter, propertyName);

            LambdaExpression call = Expression.Lambda(orderByProperty, new[] { parameter });
MethodInfo genericMethod;
            if (ascDirection)
            {
                genericMethod = OrderByMethod.MakeGenericMethod
                (new[] { typeof(TSource), orderByProperty.Type });
            }
            else
            {
                genericMethod = OrderByDescendingMethod.MakeGenericMethod
               (new[] { typeof(TSource), orderByProperty.Type });
            }
            object ret = genericMethod.Invoke(null, new object[] { source, call });
            return (IQueryable<TSource>)ret;
        }

Answer 1

问题在于：

ConditionalExpression t = Expression.IfThenElse(call, Expression.Constant(2), Expression.Constant(1));

成为Action类型的表达式，执行“then”和“else”的表达式，但不会按预期返回值。这与OrderBy方法所需的签名不匹配，因此它会抛出您看到的异常。你想要这个：

ConditionalExpression t = Expression.Condition(call, Expression.Constant(2), Expression.Constant(1));

Condition节点类型具有返回值。调试它的一个好方法是编写一个执行此操作的单元测试：

Expression<Func<bool,int>> expr = (a) => a ? 2 : 1

然后使用调试器检查生成的表达式树。希望有所帮助。

执行LambdaExpression时遇到麻烦

1 个答案: