Compact Framework - how do I dynamically create type with no default constructor?
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09-06-2019 - |
Question
I'm using the .NET CF 3.5. The type I want to create does not have a default constructor so I want to pass a string to an overloaded constructor. How do I do this?
Code:
Assembly a = Assembly.LoadFrom("my.dll");
Type t = a.GetType("type info here");
// All ok so far, assembly loads and I can get my type
string s = "Pass me to the constructor of Type t";
MyObj o = Activator.CreateInstance(t); // throws MissMethodException
Solution
MyObj o = null;
Assembly a = Assembly.LoadFrom("my.dll");
Type t = a.GetType("type info here");
ConstructorInfo ctor = t.GetConstructor(new Type[] { typeof(string) });
if(ctor != null)
o = ctor.Invoke(new object[] { s });
OTHER TIPS
@Jonathan Because the Compact Framework has to be as slim as possible. If there's another way to do something (like the code I posted) then they generally don't duplicate the functionality.
Rory Blyth once described the Compact Framework as "a wrapper around System.NotImplementedExcetion". :)
Ok, here's a funky helper method to give you a flexible way to activate a type given an array of parameters:
static object GetInstanceFromParameters(Assembly a, string typeName, params object[] pars)
{
var t = a.GetType(typeName);
var c = t.GetConstructor(pars.Select(p => p.GetType()).ToArray());
if (c == null) return null;
return c.Invoke(pars);
}
And you call it like this:
Foo f = GetInstanceFromParameters(a, "SmartDeviceProject1.Foo", "hello", 17) as Foo;
So you pass the assembly and the name of the type as the first two parameters, and then all the constructor's parameters in order.
See if this works for you (untested):
Type t = a.GetType("type info here");
var ctors = t.GetConstructors();
string s = "Pass me to the ctor of t";
MyObj o = ctors[0].Invoke(new[] { s }) as MyObj;
If the type has more than one constructor then you may have to do some fancy footwork to find the one that accepts your string parameter.
Edit: Just tested the code, and it works.
Edit2: Chris' answer shows the fancy footwork I was talking about! ;-)