How can I create a dynamic multi-property Select on an IEnumerable<T> at runtime?
c#, duck-typing, lambda, linq
Solution
There is no easy way to do exactly what you want, because it would require you to create a new type dynamically (anonymous types are created by the compiler when they're known statically). While it is certainly feasible, it's probably not the easiest option...
You can achieve a similar result using tuples:
public Expression<Func<TItem, object>> SelectExpression<TItem>(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<Func<TItem, object>>(body, param);
return expr;
}
Problem
I asked a very similar question yesterday, but it wasn't until today I realised the answer I accepted doesn't solve all my problems. I have the following code: ``` public Expression<Func<TItem, object>> SelectExpression<TItem>(string fieldName) { var param = Expression.Parameter(typeof(TItem), "item"); var field = Expression.Property(param, fieldName); return Expression.Lambda<Func<TItem, object>>(field, new ParameterExpression[] { param }); } ``` Which is used as follows: ``` string primaryKey = _map.GetPrimaryKeys(typeof(TOriginator)).Single(); var primaryKeyExpression = SelectExpression<TOriginator>(primaryKey); var primaryKeyResults = query.Select(primaryKeyExpression).ToList(); ``` This allows me to pull out the primary keys from an `IQueryable<TUnknown>`. The problem is that this code only works with a single primary key and I need to add support for multiple PKs. So, is there any way I can adapt the `SelectExpression` method above to take an `IEnumerable<string>` (which is my list of primary key property names) and have the method return an expression that selects those keys? I.e. Given the following: ``` var knownRuntimePrimaryKeys = new string[] { "CustomerId", "OrderId" }` ``` My Select needs to do the following (at runtime): ``` var primaryKeys = query.Select(x=> new { x.CustomerId, x.OrderId }); ```