在C＃中创建动态查询

Question

I am creating a custom reporting tool that allows the user to enter a list of criteria.

public partial class CustomReportCriteria
    {
        public int Id { get; set; }
        public int ReportId { get; set; }
        public string Operator { get; set; }
        public string CriteriaValue { get; set; }
        public string CriteriaType { get; set; }
        public string CriteriaField { get; set; }
        public string PropertyType { get; set; }

        public virtual CustomReport CustomReport { get; set; }
    }

I need to utilize these fields to create dynamic queries on my database. For example:

Select BidYear From FWOBid Where BidYear = 2015
// ^ this would look like this instead
Select CriteriaField From PropertyType Where CriteriaField [Operator] CriteriaValue

However, I may not always be querying the same table, and in most cases the query will require joins on other tables.

I already have generic methods for the where clauses:

 public static IQueryable<T> Filter<T>(IQueryable<T> query, string propertyName, string propertyValue, string op)
    {
        PropertyInfo propertyInfo = typeof(T).GetProperty(propertyName);
        return Filter<T>(query, propertyInfo, propertyValue, op);
    }

    public static IQueryable<T> Filter<T>(IQueryable<T> query, PropertyInfo propertyInfo, string propertyValue, string op)
    {
        ParameterExpression e = Expression.Parameter(typeof(T), "e");
        MemberExpression m = Expression.MakeMemberAccess(e, propertyInfo);
        UnaryExpression c = GetExpressionByType(propertyValue, propertyInfo.PropertyType);

        BinaryExpression b = null;
        if (op == "=")
        {
            b = Expression.Equal(m, c);
        }
        else if (op == "!=")
        {
            b = Expression.NotEqual(m, c);
        }
        else if (op == ">")
        {
            b = Expression.GreaterThan(m, c);
        }
        else if (op == "<")
        {
            b = Expression.LessThan(m, c);
        }

        Expression<Func<T, bool>> lambda = Expression.Lambda<Func<T, bool>>(b, e);
        return query.Where(lambda);
    }

The approach I was envisioning was something like this:

foreach (var crit in report.CustomReportCriterias)
            {
                var objectName = crit.PropertyType;
                var value = crit.CriteriaValue;
                var type = crit.CriteriaType;
                var field = crit.CriteriaField;
                var op = crit.Operator;

                // how to dynamically get a generic collection ?
                var queryable = _context.Set<objectName>();
                var results = Filter<objectName>(queryable, field, value, op);
                // would then do joins if needed
            }

But, I'm struggling with the initial steps of retrieving a queryable from the db based on a string, and then I'm lost as how to join these results after.

Answer 1

Have a look at https://dynamiclinq.codeplex.com/releases/view/109593 . This library extends the Linq library with a System.Linq.Dynamic namespace that provides string-based overloads to common query methods, so you can dynamically build string-based criteria for filtering and grouping. In addition, the one in my link is one better than the version available through NuGet, because it allows you to define the entire query, including the record type to retrieve, in Linq syntax as one string (see the Documentation tab for one example; it uses a static typeof expression to provide a Type to parse the results into, but you can reflectively create generic types with Type.MakeGenericType() ).

The problem's going to be getting back to a statically-defined type; once you go dynamic, you tend to have to stay dynamic, using GetProperties() to enumerate data fields and GetValue/SetValue to manipulate them. There are a few tricks, typically requiring the use of a set of concretely-typed overloads that force the runtime to examine the true type of the object, then once the object's passed into one of those overloads you're back in static world again.

Answer 2

Using Entity Framework you can execute a query directly and have the results map to an entity like this.

var results = db.ExecuteStoreQuery<YourEntity>(
    "SELECT * FROM YourEntities WHERE SomeColumn = @param1", 
    param1);