C# Linq - 根据用户输入创建动态 select 查询

Question

Use Case: User needs to be able to select a field of input to review. This field request is then passed through the query which then returns the lists of inputs by the user for review. Basically, a dynamic Select Statement in SQL where the FROM column is a variable defined by the user.

Method: Using C# and Linq I've written the following Linq Query to return the data, however, I'm not able to swap the actual field name in the query with a variable that has been set by the user.

Research: I've reviewed a dozen or so similar questions and articles and they all seem far more complicated than what I'm trying to accomplish and they don't seem to answer the question I have fully; that or I'm just not understanding what the solution actually is. I've tried playing around with Dynamic Linq without much luck and using Expression Trees seems far more complicated that what I'm hoping is actually necessary.

Explicitly Defined Field Select:

                    var Values = conn.Table<DataTable>()
                                      .Where(t => t.User_ID == ID)
                                      .Select(t => FieldName).Distinct().ToList();

Variable Field Select

                   //allowing the user to set the value
                   string UserSelectedField = y_List[0].ToString(); 
                   var Values = conn.Table<DataTable>()
                                    .Where(t => t.User_ID == ID)
                                    .Select(t => @UserSelectedField).Distinct().ToList();

In the first query I get the results I'm looking for, but it is only able to return the values from the field typed in the code.

The Second returns the object name like {FieldName = FieldName}.

I've also tried just concatenating this in the query, but that didn't work. Figured it wouldn't, but was worth a shot.

Answer 1

You have to write own extension, limitation here that you have to know type of the field:

public static class MyExtensions
{
    public class DynamicHelper<T>
    {
        private IQueryable<T> _query;

        internal DynamicHelper(IQueryable<T> query)
        {
            _query = query;
        }

        public IQueryable<TResult> SelectByFieldName<TResult>(string fieldName)
        {
            return MyExtensions.SelectByFieldName<T, TResult>(_query, fieldName);
        }
    }

    public static DynamicHelper<T> Dynamic<T>(this IQueryable<T> query)
    {
        return new DynamicHelper<T>(query);
    }

    private static IQueryable<TResult> SelectByFieldName<T, TResult>(IQueryable<T> query, string fieldName)
    {
        var param = Expression.Parameter(typeof(T), "e");
        Expression body = Expression.PropertyOrField(param, fieldName);
        if (body.Type != typeof(TResult))
            body = Expression.Convert(body, typeof(TResult));
        var lambda = Expression.Lambda<Func<T, TResult>>(body, param);
        return query.Select(lambda);
    }
}

And usage

string UserSelectedField = y_List[0].ToString(); 
var Values = conn.Table<DataTable>()
   .Where(t => t.User_ID == ID)
   .Dynamic().SelectByFieldName<string>(UserSelectedField)
   .Distinct()
   .ToList();