How to create LINQ Expression Tree to select an anonymous type
c# entity-framework expression-trees linq linq-to-entities
Question
I would like to generate the following select statement dynamically using expression trees:
var v = from c in Countries
where c.City == "London"
select new {c.Name, c.Population};
I have worked out how to generate
var v = from c in Countries
where c.City == "London"
select new {c.Name};
but I cannot seem to find a constructor/overload that will let me specify multiple properties in my select lambda.
Accepted Answer
This can be done, as mentioned, with the help of Reflection Emit and a helper class I've included below. The code below is a work in progress, so take it for what it's worth... 'it works on my box'. The SelectDynamic method class should be tossed in a static extension method class.
As expected, you won't get any Intellisense since the type isn't created until runtime. Works good on late-bound data controls.
public static IQueryable SelectDynamic(this IQueryable source, IEnumerable<string> fieldNames)
{
Dictionary<string, PropertyInfo> sourceProperties = fieldNames.ToDictionary(name => name, name => source.ElementType.GetProperty(name));
Type dynamicType = LinqRuntimeTypeBuilder.GetDynamicType(sourceProperties.Values);
ParameterExpression sourceItem = Expression.Parameter(source.ElementType, "t");
IEnumerable<MemberBinding> bindings = dynamicType.GetFields().Select(p => Expression.Bind(p, Expression.Property(sourceItem, sourceProperties[p.Name]))).OfType<MemberBinding>();
Expression selector = Expression.Lambda(Expression.MemberInit(
Expression.New(dynamicType.GetConstructor(Type.EmptyTypes)), bindings), sourceItem);
return source.Provider.CreateQuery(Expression.Call(typeof(Queryable), "Select", new Type[] { source.ElementType, dynamicType },
Expression.Constant(source), selector));
}
public static class LinqRuntimeTypeBuilder
{
private static readonly ILog log = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
private static AssemblyName assemblyName = new AssemblyName() { Name = "DynamicLinqTypes" };
private static ModuleBuilder moduleBuilder = null;
private static Dictionary<string, Type> builtTypes = new Dictionary<string, Type>();
static LinqRuntimeTypeBuilder()
{
moduleBuilder = Thread.GetDomain().DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.Run).DefineDynamicModule(assemblyName.Name);
}
private static string GetTypeKey(Dictionary<string, Type> fields)
{
//TODO: optimize the type caching -- if fields are simply reordered, that doesn't mean that they're actually different types, so this needs to be smarter
string key = string.Empty;
foreach (var field in fields)
key += field.Key + ";" + field.Value.Name + ";";
return key;
}
public static Type GetDynamicType(Dictionary<string, Type> fields)
{
if (null == fields)
throw new ArgumentNullException("fields");
if (0 == fields.Count)
throw new ArgumentOutOfRangeException("fields", "fields must have at least 1 field definition");
try
{
Monitor.Enter(builtTypes);
string className = GetTypeKey(fields);
if (builtTypes.ContainsKey(className))
return builtTypes[className];
TypeBuilder typeBuilder = moduleBuilder.DefineType(className, TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.Serializable);
foreach (var field in fields)
typeBuilder.DefineField(field.Key, field.Value, FieldAttributes.Public);
builtTypes[className] = typeBuilder.CreateType();
return builtTypes[className];
}
catch (Exception ex)
{
log.Error(ex);
}
finally
{
Monitor.Exit(builtTypes);
}
return null;
}
private static string GetTypeKey(IEnumerable<PropertyInfo> fields)
{
return GetTypeKey(fields.ToDictionary(f => f.Name, f => f.PropertyType));
}
public static Type GetDynamicType(IEnumerable<PropertyInfo> fields)
{
return GetDynamicType(fields.ToDictionary(f => f.Name, f => f.PropertyType));
}
}
Popular Answer
The accepted answer is very useful, but I needed something a little closer to a real anonymous type.
A real anonymous type has read-only properties, a constructor for filling in all of the values, an implementation of Equals/GetHashCode for comparing the values of each property, and an implementation ToString that includes the name/value of each property. (See https://msdn.microsoft.com/en-us/library/bb397696.aspx for a full description of anonymous types.)
Based on that definition of anonymous classes, I put a class that generates dynamic anonymous types on github at https://github.com/dotlattice/LatticeUtils/blob/master/LatticeUtils/AnonymousTypeUtils.cs. The project also contains some unit tests to make sure the fake anonymous types behave like real ones.
Here's a very basic example of how to use it:
AnonymousTypeUtils.CreateObject(new Dictionary<string, object>
{
{ "a", 1 },
{ "b", 2 }
});
Also, another note: I found that when using a dynamic anonymous type with Entity Framework, the constructor must be called with the "members" parameter set. For example:
Expression.New(
constructor: anonymousType.GetConstructors().Single(),
arguments: propertyExpressions,
members: anonymousType.GetProperties().Cast<MemberInfo>().ToArray()
);
If you used one of the versions of Expression.New that does not include the "members" parameter, Entity Framework would not recognize it as the constructor of an anonymous type. So I assume that means a real anonymous type's constructor expression would include that "members" information.