动态linq表达式查询，出现问题

Question

I an tryin to join 4 tables within a query as per requirement. where as I wanted to add the conditions in where clause dynamically so, i could able to do this for 2 table query as of now. but this 4 table join is bit the complex join here. To extend the functionality i am using following code to add dynamic where clause :

public static class Extensions
    {
        public static IQueryable<T> AddEqualityCondition<T, V>(this IQueryable<T> queryable,
          string propertyName, V propertyValue)
        {
            ParameterExpression pe = Expression.Parameter(typeof(T), "p");

   IQueryable<T> x = queryable.Where<T>(Expression.Lambda<Func<T, bool>>(Expression.Equal(Expression.Property(pe, typeof(T).GetProperty(propertyName)), Expression.Constant(propertyValue, typeof(V)), false, typeof(T).GetMethod("op_Equality")), new ParameterExpression[] { pe }));
            return (x);
        }
    }

// My code to add where conditions:

Query is:
 var agrs = (from agr in _dbContext.Agreements
                                 join amdv in _dbContext.AgreementMetaDataValues on agr.AgreementID equals amdv.AgreementID 
                                 join emd in _dbContext.EntityMetadatas on amdv.AttributeId equals emd.AttributeId
                                 join et in _dbContext.Entities on agr.EntityID equals et.EntityId
                                 select new  agr, amdv,emd });

//Add dynamically where conditions:
 agrs = agrs.AddEqualityCondition("?????", "A83C82C5-F9D6-4833-A234-EBB5D971280C");

This is working for 2 table join not for more than that. because within complex query it is generating the Annonymouse object. so so what should i need to pass in place of "??????" marks...? typically need to pass the property name as"agr.AgreementId" but here it is throwing the expression as "Value Canot be Null : propertyName" in extension class. Need more guidance for this ...

Answer 1

I think you might want to consider something like (as an additional overload):

public static IQueryable<T> AddEqualityCondition<T, V>(
    this IQueryable<T> queryable,
    Expression<Func<T, V>> selector, V propertyValue)
{
    var lambda = Expression.Lambda<Func<T,bool>>(
       Expression.Equal(
           selector.Body,
           Expression.Constant(propertyValue, typeof(V)),
           false, typeof(T).GetMethod("op_Equality")),
        selector.Parameters);
    return queryable.Where(lambda);           
}

and using:

agrs = agrs.AddEqualityCondition(x => x.agr.AgreementId, 
             "A83C82C5-F9D6-4833-A234-EBB5D971280C");

however! it is much easier to use just:

agrs = agrs.Where(x => x.agr.AgreementId ==
             "A83C82C5-F9D6-4833-A234-EBB5D971280C");

Answer 2

最好使用谓词构建器，即动态组合表达式谓词 ，它使您可以轻松地动态构建查询。