CreateInstance from Type name and Assembly name

Question

I'm trying to create a class using Activator.CreateInstance(assemblyName, typeName) but I am getting the error

An unhandled exception of type 'System.TypeLoadException' occurred in mscorlib.dll

Additional information: Could not load type 'Question' from assembly 'AssessmentDeepCompare, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

In this example (which generates the error above) I'm trying to instantiate Question which inherits from IAssessmentObject and is in the same assembly.

public IAssessmentObject getInstance(string typeName) {
    string assemblyName = Assembly.GetAssembly(typeof (IAssessmentObject)).ToString();
    return (IAssessmentObject)Activator.CreateInstance(assemblyName, typeName);
}

What is the proper way to instantiate an object when you only have the Type name without the namespace or assembly?

Answer 1

It sounds like you do have the assembly - just not the namespace-qualified type name.

You can fetch all the types in the assembly, and get the type from that:

var type = typeof(IAssessmentObject).Assembly
                                    .GetTypes()
                                    .Single(t => t.Name == "Question");
var instance = (IAssessmentObject) Activator.CreateInstance(type);

Answer 2

If you have the assembly name and the type name, as well as the folder they live in, you can do this. I built this to return a logging class based on choices in the config file.

string _assemblyName = ConfigurationManager.AppSettings["MailLogger.AssemblyName"];
string _className = ConfigurationManager.AppSettings["MailLogger.ClassName"];
string _assemblyPath = ConfigurationManager.AppSettings["Reporting.Assembly.Folder"];

if (string.IsNullOrWhiteSpace(_assemblyName) || string.IsNullOrWhiteSpace(_className))
    throw new ApplicationException("Missing configuration data for the Logger.");

Assembly _loggingAssembly = Assembly.LoadFrom(System.IO.Path.Combine(_assemblyPath, _assemblyName));
ILogger _logger = _loggingAssembly.CreateInstance(_className) as ILogger;

if (_logger == null)
    throw new ApplicationException(
            string.Format("Unable to instantiate ILogger instance from {0}/{1}", _assemblyName, _className));

return _logger;

If you don't have the assembly name, I'm not sure how would create it via reflection.