扩展IQueryable的Where方法 [英] Extend the Where method for IQueryable

问题描述

我想将 Where 扩展到这样的东西： .WhereEqual（Field，SomeString） .Where（p => p.Field ==SomeString），这里是我的代码：

I want to extend the Where to something like this: .WhereEqual("Field", "SomeString") wich must be equivalent to .Where(p => p.Field== "SomeString"), here is my code:

public static IQueryable<T> WhereEqual<T>(this IQueryable<T> q, string Field, string Value)
    {
        var param = Expression.Parameter(typeof(T), "p");
        var prop = Expression.Property(param, Field);

        var val = Expression.Constant(Value);
        var body = Expression.Equal(prop, val);

        var exp = Expression.Lambda<Func<T, bool>>(body, param);

        Type[] types = new Type[] { q.ElementType, typeof(bool) };

        var mce = Expression.Call(
                typeof(Queryable),
                "Where",
                types,
                exp
            );
        return q.Provider.CreateQuery<T>(mce);
    }

感谢调试器我可以看到 exp 设置为 {p => （p.Field ==SomeString）} ，但调用方法抛出以下异常：

thanks to the debugger i can see that exp is set to {p => (p.Field== "SomeString")}, but the Call method throws the following exception :

System.Core.dll中出现System.InvalidOperationException类型的异常，但未在用户代码中处理，附加
信息：否通用方法'Where'on type'System.Linq.Queryable'
与提供的类型参数和参数兼容，如果方法非泛型，则不应该提供类型
参数。

"An exception of type 'System.InvalidOperationException' occurred in System.Core.dll but was not handled in user code, Additional information: No generic method 'Where' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic."

如何给哪里需要的参数？

谢谢。

推荐答案

public static IQueryable<T> WhereEqual<T>(this IQueryable<T> q, string Field, string Value)
    {
        var param = Expression.Parameter(typeof(T), "p");
        var prop = Expression.Property(param, Field);

        var val = Expression.Constant(Value);
        var body = Expression.Equal(prop, val);

        var exp = Expression.Lambda<Func<T, bool>>(body, param);


        return System.Linq.Queryable.Where(q, exp);
    }

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