Activator.CreateInstance failing with 'No parameterless constructor'

Question

I'm creating a method that will use CastleWindsor to try to resolve a type, but use a default type if the component isn't configured (so I don't have to configure everything until I actually want to change the implementation). Here's my method...

public static T ResolveOrUse<T, U>() where U : T
    {
        try
        {
            return container.Resolve<T>();
        }
        catch (ComponentNotFoundException)
        {
            try
            {
                U instance = (U)Activator.CreateInstance(typeof(U).GetType());
                return (T)instance;
            }
            catch(Exception ex)
            {
                throw new InvalidOperationException("IOC Couldn't instantiate a '" + typeof(U) + "' because: " + ex.Message);
            }
        }
    }

When a WebConfigReader is passed in as the default type to use, I get the error "No parameterless constructor defined for this object". Here's my WebConfigReader class...

public class WebConfigReader : IConfigReader
{
    public string TfsUri
    {
        get { return ReadValue<string>("TfsUri"); }
    }

    private T ReadValue<T>(string configKey)
    {
        Type type = typeof(T).GetType();
        return (T)Convert.ChangeType(ConfigurationManager.AppSettings[configKey], type);
    }
}

Since I have no ctor, it should work. I've added a paramless ctor and I've passed in true as the second param to CreateInstance and none of the above worked. I can't figure out what I'm missing. Any thoughts?

WebConfigReader would not compile as is, what does your real code look like? — BrokenGlass
– BrokenGlass, Commented Feb 5, 2012 at 2:44
@Baboon It's not redundant; it's incorrect. typeof(U) returns an object that represents the type U, but the type of that object is System.Type, so the type object returned by the GetType call is the one that represents System.Type, not the one that represents U. — phoog
– phoog, Commented Feb 5, 2012 at 2:56
@phoog that's what i wanted to say, but it was late at night.. — Louis Kottmann
– Louis Kottmann, Commented Feb 5, 2012 at 14:08

M.Babcock · Accepted Answer · 2012-12-01 04:32:53Z

11

typeof(U) will already return the type U represents. Doing the extra GetType() on it will return the type System.Type which doesn't have a default constructor.

So your first code block could be written as:

public static T ResolveOrUse<T, U>() where U : T
{
    try
    {
        return container.Resolve<T>();
    }
    catch (ComponentNotFoundException)
    {
        try
        {
            U instance = (U)Activator.CreateInstance(typeof(U));
            return (T)instance;
        }
        catch(Exception ex)
        {
            throw new InvalidOperationException("IOC Couldn't instantiate a '" + typeof(U) + "' because: " + ex.Message);
        }
    }
}

edited Dec 1, 2012 at 4:32

answered Feb 5, 2012 at 2:48

M.Babcock

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3 Comments

BrokenGlass Over a year ago

same exact mistake also in the ReadValue<T>() method - just use typeof(U)

Gabe Over a year ago

I think the downvote was because you didn't provide an answer. You said what the OP doesn't want, but not what the OP does want.

Gabe Over a year ago

@M.Babcock: That's my guess, and I was probably right since you edited the post and the downvote went away. In general, though, "don't do X" should be a comment; "do Y instead of X" should be an answer.

JaredPar · Accepted Answer · 2012-02-05 02:59:31Z

5

Since you have a generic type parameter you should just use the generic overload of Activator.CreateInstance

U instance = Activator.CreateInstance<U>();

answered Feb 5, 2012 at 2:59

JaredPar

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2 Comments

Gabe Over a year ago

Do you have any idea why the generic version only supports parameterless constructors?

JaredPar Over a year ago

@Gabe I don't know for sure. But one of it's main purposes is supporting the new() constraint so it only needs to support parameterles ctors. Not being able to specify a constructor with arguments is likely a factor in why CreateInstance can't eiter

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