每个属性的GroupBy Lamda表达式生成器

Question

I want to populate the itemsource of a ComboBox with items from my List, depending on which property from T is selected.

The statement should be like: foreach property which is a string, select the values of the property, make distinct.

    public Dictionary<string, List<string>> CreateSuggestionsLists<T>(List<T> data)
    {
        var queryableData = data.AsQueryable();
        var paramExp = Expression.Parameter(typeof(T), "left");
        foreach (var pi in typeof(T).GetProperties().Where(p => p.PropertyType == typeof(string)))
        {
            var callExpr = Expression.MakeMemberAccess(paramExp, pi);
            var lambdaExpr = Expression.Lambda(callExpr) ;
            // From here on it goes wrong!!!
            var comleteExpr = lambdaExpr as Expression<Func<T, bool>>;
            var compiledExpr = comleteExpr.Compile();

            var res = data.Select(compiledExpr).Distinct().ToList();
            // add to results ...
        }

        return null;
    }

The problem seems to be the casting from the lambda expression to prepare for compilation.

Thank you for your help.

Answer 1

First of all you need to provide paramExp to lambda. Secondly there is generic version of Lamda method which is just easier to use. Finally, you don't need to compile expression when you use IQueryable . You created queryableData variable and didn't use it.

Here is code:

    public Dictionary<string, List<string>> CreateSuggestionsLists<T>(List<T> data)
    {
        var queryableData = data.AsQueryable();
        var paramExp = Expression.Parameter(typeof(T), "left");
        foreach (var pi in typeof(T).GetProperties().Where(p => p.PropertyType == typeof(string)))
        {
            var callExpr = Expression.MakeMemberAccess(paramExp, pi);
            var lambdaExpr = Expression.Lambda<Func<T, bool>>(callExpr, paramExp);

            var res = queryableData.Select(lambdaExpr).Distinct().ToList();
            // add to results ...
        }

        return null;
    }

Answer 2

I think you should check if the casting result is not null :

          public Dictionary<string, List<string>> CreateSuggestionsLists<T>(List<T> data)
    {
        IQueryable<T> queryableData = data.AsQueryable();
        ParameterExpression paramExp = Expression.Parameter(typeof(T), "left");

        foreach (PropertyInfo pi in typeof(T).GetProperties().Where(p => p.PropertyType == typeof(string)))
        {
            MemberExpression callExpr = Expression.MakeMemberAccess(paramExp, pi);
            LambdaExpression lambdaExpr = Expression.Lambda(callExpr);
            // From here on it goes wrong!!!
            if (!(lambdaExpr is Expression<Func<T, bool>> comleteExpr)) continue;

            Func<T, bool> compiledExpr =  comleteExpr.Compile();

            List<bool> res = data.Select(compiledExpr).Distinct().ToList();

            // add to results ...
        }

        return null;
    }