Value cannot be null. Parameter name: type

Question

After having read and tried the answers on stackExchange I am still getting a null value for result in the below code:

    typeOfIMydServiceBase.Assembly
            .GetExportedTypes()
            .Where(exportedType => typeOfMydServiceBase != exportedType && typeOfMydServiceBase.IsAssignableFrom(exportedType))
            .Select(mydServiceType => new
            {
                serviceType = mydServiceType,
                interfaceType = mydServiceType.GetInterfaces().First(@interface => @interface != typeOfIMydServiceBase && typeOfIMydServiceBase.IsAssignableFrom(@interface))
            })
            .ToList()
            .ForEach(registrationInfo => container.Register(registrationInfo.interfaceType, () =>
            {
                var method = serviceActivatorType
                    .GetMethod("GetService", serviceActivatorParams)
                    .MakeGenericMethod(new[] { registrationInfo.interfaceType });
                var result = method.Invoke(new ServiceActivator(), null);

                return result;
            }, WebApiRequestLifestyleWithDisposal));

result keeps coming back as null and when I use an Object[] {null} or Object[]{} or Object[]{null,null,etc} I get parameter mismatches.

Note: this is the error message I recieve for the above code:

Line 156: .MakeGenericMethod(new[] { registrationInfo.interfaceType });

Line 157:

Line 158: var result = method.Invoke(new ServiceActivator(), null);

Line 159:

Line 160: return result;

Does anyone know what's happening here and how I can fix it????

{
public class ServiceActivator
{
    public ServiceActivator();
    public ServiceActivator(IServiceConfigurationReader configurationReader);

    public T GetService<T>();
    public T GetService<T>(string key);
}

}

Also pertinent to the above is the registration of the services used. See below:

//Scan and register business servics
        var typeOfIMydServiceBase = typeof(IMydServiceBase);
        var typeOfMydServiceBase = typeof(MydServiceBase);
        var serviceActivatorType = typeof(ServiceActivator);
        var serviceActivatorParams = new Type[0];

Update: using typeof(string) at serviceActivator Params works however I am getting an unhandled exception futher down. Here is the new error:

Line 166:                    p.ServiceContainer = container;
Line 167:                });
Line 168:
Line 169:                container.RegisterWebApiControllers(config);
Line 170:            }

which pertains to the following code:

var typeOfIMydServiceBase = typeof(IMydServiceBase);
        var typeOfMydServiceBase = typeof(MydServiceBase);
        var serviceActivatorType = typeof(ServiceActivator);
        var serviceActivatorParams = new Type[] { typeof(string) };
        
        typeOfIMydServiceBase.Assembly
            .GetExportedTypes()
            .Where(exportedType => typeOfMydServiceBase != exportedType && typeOfMydServiceBase.IsAssignableFrom(exportedType))
            .Select(mydServiceType => new
            {
                serviceType = mydServiceType,
                interfaceType = mydServiceType.GetInterfaces().First(@interface => @interface != typeOfIMydServiceBase && typeOfIMydServiceBase.IsAssignableFrom(@interface))
            })
            .ToList()
            .ForEach(registrationInfo => container.Register(registrationInfo.interfaceType, () =>
            {
                
                var method = serviceActivatorType
                    .GetMethod("GetService", serviceActivatorParams)
                    .MakeGenericMethod(new[] { registrationInfo.interfaceType });
           
                var result = method.Invoke(new ServiceActivator(), null);

                return result;
            }, WebApiRequestLifestyleWithDisposal));
        {

            container.RegisterInitializer<IMydServiceBase>(p =>
            {
                p.ServiceContainer = container;
            });

            container.RegisterWebApiControllers(config);
        }
    }

Answer 1

var method = serviceActivatorType .GetMethod("GetService", serviceActivatorParams)

returns public T GetService<T>(); with var serviceActivatorParams = new Type[0]; (from comment discussion in the question)

BUT this line:

var result = method.Invoke(new ServiceActivator(), null);

is going to invoke this method:

public T GetService<T>(string key);

passing null as the value for string key.

There is no parameter to be passed for public T GetService<T>();, that's why it throws an exception.

To fix your problem, try:

var serviceActivatorParams = new Type[] { typeof(string) };

or invoke your method without the null parameter:

var result = method.Invoke(new ServiceActivator());