Reflection: Check if Type will fulfill new() constraint

Question

I have a generic function with a new() constraint:

public void DoMagicStuff<T>() where t:new() {
  // do something
}

I call this function from within a loop over every type of an assembly:

foreach (var type in baseType.Assembly.GetTypes())
{
   MethodInfo method = GetMethod<LiteDbQueue>(q => q.DoMagicStuff<object>());
   CallGenericMethod(this, method, baseType, type, null);
}

This loop works fine if the Type inside the loop fits the new() constraint. But if that type does not have a new()-constructor (e.g. a static class) I receive an error

(Typename)', on 'Void DoMagicStuffT' violates the constraint of type 'T'.

I want to validate within the Loop if the Type fulfills the new() - constraint. So something like:

if (type.IsNewable()) { ... }

How can this be done?

Just for completion: The following methods are used to "cast" my types for the generic method. These work fine and are not really part of my problem:

public MethodInfo GetMethod<T>(Expression<Action<T>> expr) {
   return ((MethodCallExpression)expr.Body).Method.GetGenericMethodDefinition();
}

public object CallGenericMethod(object baseObject, MethodInfo method, Type baseType, Type entityType, object[] parameters)
{
   MethodInfo genericMethod = method.MakeGenericMethod(entityType);
   var paams = genericMethod.GetParameters();
   var retVal = genericMethod.Invoke(baseObject, parameters);
   return retVal;
}

Maybe have a look at https://stackoverflow.com/questions/4681031/how-do-i-check-if-a-type-provides-a-parameterless-constructor — uk2k05, Jun 03 '19 at 11:53
The type information also contains the available constructors, so just search for a ctor without parameters — Ronald Korze, Jun 03 '19 at 11:54

score 3 · Accepted Answer · answered Jun 03 '19 at 11:54

3

In a nutshell,

    public static bool HasDefaultConstructor<T>()
        => typeof (T).GetConstructor(Type.EmptyTypes) != null;

answered Jun 03 '19 at 11:54

Samuel Vidal

883
5
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Reflection: Check if Type will fulfill new() constraint

1 Answers1