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Type type = typeof (string);
Type[] parameterTypes = { typeof (int) };
MethodInfo method = type.GetMethod ("Substring", parameterTypes);
object[] arguments = { 2 };
object returnValue = method.Invoke ("stamp", arguments);
Console.WriteLine (returnValue); // "amp"
Because the Substring method is overloaded, we had to pass an array of parameter
types to GetMethod to indicate which version we wanted. Without the parameter
types, GetMethod would throw an AmbiguousMatchException.
The GetParameters method, defined on MethodBase (the base class for MethodInfo and
ConstructorInfo), returns parameter metadata. We can continue our previous example
as follows:
ParameterInfo[] paramList = method.GetParameters();
foreach (ParameterInfo x in paramList)
{
Console.WriteLine (x.Name); // startIndex
Console.WriteLine (x.ParameterType); // System.Int32
}
Dealing with ref and out parameters
To pass ref or out parameters, call MakeByRefType on the type before obtaining the
method. For instance, this code:
int x;
bool successfulParse = int.TryParse ("23", out x);
can be dynamically executed as follows:
object[] args = { "23", 0 };
Type[] argTypes = { typeof (string), typeof (int).MakeByRefType() };
MethodInfo tryParse = typeof (int).GetMethod ("TryParse", argTypes);
bool successfulParse = (bool) tryParse.Invoke (null, args);
Console.WriteLine (successfulParse + " " + args[1]); // True 23
This same approach works for both ref and out parameter types.
Retrieving and invoking generic methods
Explicitly specifying parameter types when calling GetMethod can be essential in disambiguating
overloaded methods. However, it’s impossible to specify generic
parameter types. For instance, consider the System.Linq.Enumerable class, which
overloads the Where method as follows:
public static IEnumerable Where
(this IEnumerable source, Func predicate);
public static IEnumerable Where
(this IEnumerable source, Func predicate);
To retrieve a specific overload, we must retrieve all methods and then manually find
the desired overload. The following query retrieves the former overload of Where:
694 | Chapter 18: Reflection and Metadata