Question

I have the code like this

public class SomeClass
{
    private static int staticField = 10;
}

The code never gets executed and staticField has default value 0.
Also the code causes MVVMlight’s SimpleIoc to throw an exception with code like this:

SimpleIoc.Default.Register<SomeClass>();

Above code causes MVVMLight to throw an exception saying

 Cannot build instance: Multiple constructors found but none marked with PreferredConstructor.

This very bizarre.
I’m using Win8 RTM x64 + VS2012 Express for Windows 8.

Answer 1

This is definitely a bug in SimpleIoc of MVVMLight. I have tried it with LinqPad and the problem is when you add a static field to the class an static ctor is added by the field initializer.

The result is that the class SomeClass has two ctors for SimpleIoc, what results in the exception you described.

A Workaround would be to add a default constructor to the class and decorate it with the PreferredConstructorAttribute but this will result in a dependency to SimpleIoc.

Other solution would be to change your static field to a constant value.

public class SomeClass
{
    private const int staticField = 10;
}

or to use an overload of the Register method to provide a factory method for instance creation.

SimpleIoc.Default.Register<SomeClass>(() => new SomeClass())

I have submitted a bug report on the MVVM Light project on CodePlex

LinqPad (Test code):

    void Main()
{
    var x = GetConstructorInfo(typeof(SomeClass));

    x.Dump();
    x.IsStatic.Dump();
}


public class PreferredConstructorAttribute : Attribute{

}
public class SomeClass{
  private static int staticField = 10;

}

private ConstructorInfo GetConstructorInfo(Type serviceType)
        {
            Type resolveTo = serviceType;


//#if NETFX_CORE
            var constructorInfos = resolveTo.GetTypeInfo().DeclaredConstructors.ToArray();
            constructorInfos.Dump();
//#else
//          var constructorInfos = resolveTo.GetConstructors();
//constructorInfos.Dump();
//#endif

            if (constructorInfos.Length > 1)
            {
                var preferredConstructorInfos
                    = from t in constructorInfos
//#if NETFX_CORE
                       let attribute = t.GetCustomAttribute(typeof (PreferredConstructorAttribute))
//#else
//                     let attribute = Attribute.GetCustomAttribute(t, typeof(PreferredConstructorAttribute))
//#endif
                       where attribute != null
                       select t;

preferredConstructorInfos.Dump();

var preferredConstructorInfo = preferredConstructorInfos.FirstOrDefault ( );
                if (preferredConstructorInfo == null)
                {
                    throw new InvalidOperationException(
                        "Cannot build instance: Multiple constructors found but none marked with PreferredConstructor.");
                }

                return preferredConstructorInfo;
            }

            return constructorInfos[0];
        }
// Define other methods and classes here

The problem is the line

var constructorInfos = resolveTo.GetTypeInfo().DeclaredConstructors.ToArray();

that returns an array with 2 ConstructorInfos both defined without the PreferredConstructorAttribute which results in the exception.