I want to create a dynamic GroupBy implementation that ignores case. I am using Expression.Call
, which allows me to pass Expressions as arguments.
There are several answers on how to create a custom comparer, but this question is about how to pass a comparer dynamically.
Here is the complete method:
public static IQueryable GroupBy(this IQueryable source, string keySelector, string elementSelector, params object[] values)
{
if (source == null) throw new ArgumentNullException("source");
if (keySelector == null) throw new ArgumentNullException("keySelector");
if (elementSelector == null) throw new ArgumentNullException("elementSelector");
LambdaExpression keyLambda = DynamicExpression.ParseLambda(source.ElementType, null, keySelector, false, values);
LambdaExpression elementLambda = DynamicExpression.ParseLambda(source.ElementType, null, elementSelector, false, values);
return source.Provider.CreateQuery(
Expression.Call(
typeof(Queryable),
"GroupBy",
new Type[] { source.ElementType, keyLambda.Body.Type, elementLambda.Body.Type },
source.Expression,
Expression.Quote(keyLambda),
Expression.Quote(elementLambda)
)
);
}
The call to Queryable.GroupBy
is created by:
Expression.Call(typeof(Queryable), "GroupBy",
new Type[] { source.ElementType, keyLambda.Body.Type, elementLambda.Body.Type },
source.Expression, Expression.Quote(keyLambda), Expression.Quote(elementLambda))
Queryable.GroupBy
allows to pass a custom IEqualityComparer
. How can I do this? Expression.Call
only allows me to pass arguments of type Expression
.
Is there any other way I can group with case ignored, by e.g. dynamically overriding GetHashCode()
of the keys?
StringComparer
can not be used here because the type is dynamic and not string. I had to elaborate on Krzysztofs answer to find a solution that worked.
First create an instance of a custom dynamic comparer DynamicCaseInsensitiveComparer<T>
(which implements IEqualityComparer) of the same type as keyLambda.Body.Type
. Since the type is provided by a variable, you have to use MakeGenericType
. Then add it in the GroupBy call:
var comparerType = typeof(DynamicCaseInsensitiveComparer<>).MakeGenericType(keyLambda.Body.Type);
var keyComparer = Activator.CreateInstance(comparerType);
return source.Provider.CreateQuery(
Expression.Call(
typeof(Queryable),
"GroupBy",
new Type[] { source.ElementType, keyLambda.Body.Type, elementLambda.Body.Type },
source.Expression,
Expression.Quote(keyLambda),
Expression.Quote(elementLambda),
Expression.Constant(keyComparer)
)
);
How to create a custom comparer has been answered in other questions, see for example IEqualityComparer for Annoymous Type
You should call it the same way, as you would normally by adding comparer in GroupBy
call:
return source.Provider.CreateQuery(
Expression.Call(
typeof(Queryable),
"GroupBy",
new Type[] { source.ElementType, keyLambda.Body.Type, elementLambda.Body.Type },
source.Expression,
Expression.Quote(keyLambda),
Expression.Quote(elementLambda),
Expression.Constant(StringComparer.InvariantCultureIgnoreCase)
)
);