Activator.CreateInstance以类为参数调用构造函数

Question

Hi I'm trying to do the following dynamically I'm using my own CreateInstance method but this has been tested with Activator.CreateInstance

IPEndPoint newObject = new IPEndPoint(IPAddress.Any, 80);

when I try to use activator I get error, cannot convert System.RuntimeType to IPAddress

    public static object CreateInstance(Type context, object[] Params)
    {
        List<Type> argTypes = new List<Type>();

        foreach (object Param in Params)
            argTypes.Add(GetType(Param));
        ConstructorInfo[] Types = context.GetConstructors();
        foreach (ConstructorInfo node in Types)
        {
            ParameterInfo[] Args = node.GetParameters();
            if (Params.Length == Args.Length)
            {
                bool[] cond = new bool[Params.Length];
                for (int i = 0; i < Params.Length; i++)
                    if (argTypes[i] == Args[i].ParameterType) cond[i] = true;
                if (cond[0] == true & cond[1] == true)
                    return node.Invoke(Params);
            }
        }
        return null;
    }

This is what the Params look like in the array [0] {Name = "IPAddress" FullName = "System.Net.IPAddress"} [1] 80

this is the calling code, prob should have provided it before so you know what I'm trying to do as you can see it parses string values that represent classes, this is why I can't use typeof or typeconstraints.

private object CreateInstance(ObjectCreationExpression Exp)
{
    object context = GetContext(Exp.Identifier); //Gets the class type
    Type t = (Type)context;
    object[] Params = GetParams(Exp.ArgumentList).ToArray();
    object newObject = Activator.CreateInstance(t, Params);
    return newObject;
}

public static object GetContext(string classname)
{

    return ParseNamespace("System.dll", classname);
}

private static object ParseNamespace(string Namespace, string classname) //Looks up class in System.dll
{
    string DotNetPath = ToolLocationHelper.GetPathToDotNetFramework(TargetDotNetFrameworkVersion.VersionLatest);
    Assembly Asm = Assembly.LoadFile(DotNetPath + @"\" + Namespace);
    Type[] Types = Asm.GetExportedTypes();
    foreach (Type Node in Types)
    {
        if (Node.Name == classname)
            return Node;
    }
    return null;
}

private List<object> GetParams(NodeCollection<ArgumentNode> Params)
{
    List<object> Arguments = new List<object>();
    foreach (ArgumentNode node in Params)
    {

        if (node.Expression is MemberAccessExpression)
        {
            MemberAccessExpression exp = (MemberAccessExpression)node.Expression;
            Type value = (Type)GetContext(exp);
            string name = DirectCast<IdentifierExpression>(exp.Right).Identifier;
            if (value.IsEnum)
            {
                string[] names = DirectCast<Type>(value).GetEnumNames();
                Array item = DirectCast<Type>(value).GetEnumValues();
                Arguments.Add(item.GetValue(names.ToList().IndexOf(name)));
            }
            else
            {
                Type item = value.GetMember(name)[0].ReflectedType;
                Arguments.Add(item);
            }
        }
        else
            Arguments.Add((Int32)ParseType(node.Expression));
    }
    return Arguments;
}

ObjectCreationExpression is a custom class that contains parsed sourcecode for creating a new instance, the two main properties are ArgumentList which is a collection of values or identifiers to be used as parameters, the other property is an identifier for the type we are creating

Answer 1

You have wrote a nice implementation to create object instance, however it had some flaws. I've corrected them in the code below

    public static object CreateInstance(Type context, params object[] Params) // params keyword for array
    {
        List<Type> argTypes = new List<Type>();

        //used .GetType() method to get the appropriate type
        //Param can be null so handle accordingly
        foreach (object Param in Params)
            argTypes.Add((Param ?? new object()).GetType());
        ConstructorInfo[] Types = context.GetConstructors();
        foreach (ConstructorInfo node in Types)
        {
            ParameterInfo[] Args = node.GetParameters();
            if (Params.Length == Args.Length)
            {
                bool[] cond = new bool[Params.Length];
                //handle derived types
                for (int i = 0; i < Params.Length; i++)
                    if (Args[i].ParameterType.IsAssignableFrom(argTypes[i])) cond[i] = true;
                if (cond[0] && cond[1])
                    return node.Invoke(Params);
            }
        }
        return null;
    }

• paramaters were not an array
• Param.GetType() is more appropriate
• handle parameter of derived types(maybe buggy at this moment as value types and class type need to be differentiated)

calling code

IPEndPoint newObject = (IPEndPoint)CreateInstance(typeof(IPEndPoint), IPAddress.Any, 80);

Note I may not be able to correct every flaw in the sample above, I just made it workable for your scenario ie you calling code

Generics implementation

    public static T CreateInstance<T>(params object[] Params) where T : class // params keyword for array
    {
        List<Type> argTypes = new List<Type>();

        //used .GetType() method to get the appropriate type
        //Param can be null so handle accordingly
        foreach (object Param in Params)
            argTypes.Add((Param ?? new object()).GetType());
        ConstructorInfo[] Types = typeof(T).GetConstructors();
        foreach (ConstructorInfo node in Types)
        {
            ParameterInfo[] Args = node.GetParameters();
            if (Params.Length == Args.Length)
            {
                bool[] cond = new bool[Params.Length];
                //handle derived types
                for (int i = 0; i < Params.Length; i++)
                    if (Args[i].ParameterType.IsAssignableFrom(argTypes[i])) cond[i] = true;
                if (cond[0] && cond[1])
                    return (T)node.Invoke(Params);
            }
        }
        return default(T);
    }

calling code

IPEndPoint newObject = CreateInstance<IPEndPoint>(IPAddress.Any, 80);

Fully dynamic object construction

    public static object CreateInstance(Type pContext, object[] Params)
    {
        List<Type> argTypes = new List<Type>();

        //used .GetType() method to get the appropriate type
        //Param can be null so handle accordingly
        if (Params != null)
            foreach (object Param in Params)
            {
                if (Param != null)
                    argTypes.Add(Param.GetType());
                else
                    argTypes.Add(null);
            }

        ConstructorInfo[] Types = pContext.GetConstructors();
        foreach (ConstructorInfo node in Types)
        {
            ParameterInfo[] Args = node.GetParameters();
            // Params can be null for default constructors so use argTypes
            if (argTypes.Count == Args.Length)
            {
                bool areTypesCompatible = true;
                for (int i = 0; i < Params.Length; i++)
                {
                    if (argTypes[i] == null)
                    {
                        if (Args[i].ParameterType.IsValueType)
                        {
                            //fill the defaults for value type if not supplied
                            Params[i] = CreateInstance(Args[i].ParameterType, null);
                            argTypes[i] = Params[i].GetType();
                        }
                        else
                        {
                            argTypes[i] = Args[i].ParameterType;
                        }
                    }
                    if (!Args[i].ParameterType.IsAssignableFrom(argTypes[i]))
                    {
                        areTypesCompatible = false;
                        break;
                    }
                }
                if (areTypesCompatible)
                    return node.Invoke(Params);
            }
        }

        //delegate type to Activator.CreateInstance if unable to find a suitable constructor
        return Activator.CreateInstance(pContext);
    }

calling code

IPEndPoint newObject = (IPEndPoint)CreateInstance(typeof(IPEndPoint), new object[] { IPAddress.Any, 80});

this code can also null parameters

eg

IPEndPoint newObject = (IPEndPoint)CreateInstance(typeof(IPEndPoint), new object[] { IPAddress.Any, null});

I simplified it a bit and also handled null parameters for default constructors also. and couple of other checks

so this change makes it complete dynamic even you can construct value types too

eg

int obj = (int)CreateInstance(typeof(int), null);

Example for your case

object context = GetContext(Exp.Identifier);
Type t = (Type)context;
object[] Params = GetParams(Exp.ArgumentList).ToArray();
//use the above defined method and it will work as expected
object newObject = CreateInstance(t, Params);

Answer 2

For what it's worth, this is my refactoring of your method:

public static object CreateInstance(Type pContext, params object[] pArguments) {
   var constructors = pContext.GetConstructors();

   foreach (var constructor in constructors) {
      var parameters = constructor.GetParameters();
      if (parameters.Length != pArguments.Length)
         continue;

      // assumed you wanted a matching constructor
      // not just one that matches the first two types
      bool fail = false;
      for (int x = 0; x < parameters.Length && !fail; x++)
         if (!parameters[x].ParameterType.IsInstanceOfType(pArguments[x]))
            fail = true;

      if (!fail)
         return constructor.Invoke(pArguments);
   }
   return null;
}

Note that you seem to have the notion of "parameter" and "argument" backwards. A "parameter" is the named part of the method that accepts a value. An "argument" is the actual value that you pass.

Aside from that, it sounds like your problem has more to do with the values you are passing, than the implementation of the method.