Build GroupBy expression tree with multiple fields

Question

To dynamically generate a GroupBy expression, I am trying to build a Linq expression tree. The fields to group by are dynamic and can differ in number.

I use this co

Answer 1

This post shows a expression function which can be used for both Select and GroupBy. Hope it helps others!

public Expression> GroupByExpression(string[] propertyNames)
{
    var properties = propertyNames.Select(name => typeof(TItem).GetProperty(name)).ToArray();
    var propertyTypes = properties.Select(p => p.PropertyType).ToArray();
    var tupleTypeDefinition = typeof(Tuple).Assembly.GetType("System.Tuple`" + properties.Length);
    var tupleType = tupleTypeDefinition.MakeGenericType(propertyTypes);
    var constructor = tupleType.GetConstructor(propertyTypes);
    var param = Expression.Parameter(typeof(TItem), "item");
    var body = Expression.New(constructor, properties.Select(p => Expression.Property(param, p)));
    var expr = Expression.Lambda>(body, param);
    return expr;
}  

To be called like this:

var lambda = GroupByExpression(fields);
var currentItemFields = students.GroupBy(lambda.Compile());