LuckyPennySoftware/AutoMapper · error · InvalidOperationException

As must specify a derived type, not {DestinationType}

Error message

As must specify a derived type, not {DestinationType}

What it means

Thrown by the As(Type) method when the type argument equals the destination type itself. As is designed to override the destination with a derived type for polymorphic dispatch, so passing the same type is meaningless. After this check, CheckIsDerivedFrom validates that typeOverride actually derives from DestinationType.

Source

Thrown at src/AutoMapper/Configuration/TypeMapConfiguration.cs:237

        {
            if (config.DestinationMember == destinationMember)
            {
                return config;
            }
        }
        return null;
    }
    protected abstract void IgnoreDestinationMember(MemberInfo property, bool ignorePaths = true);
}
public abstract class MappingExpressionBase<TSource, TDestination, TMappingExpression>(MemberList memberList, TypePair types) : TypeMapConfiguration(memberList, types), IMappingExpressionBase<TSource, TDestination, TMappingExpression>
    where TMappingExpression : class, IMappingExpressionBase<TSource, TDestination, TMappingExpression>
{
    protected MappingExpressionBase(MemberList memberList) : this(memberList, new(typeof(TSource), typeof(TDestination))) { }
    public void As(Type typeOverride)
    {
        if (typeOverride == DestinationType)
        {
            throw new InvalidOperationException("As must specify a derived type, not " + DestinationType);
        }
        typeOverride.CheckIsDerivedFrom(DestinationType);
        DestinationTypeOverride = typeOverride;
    }
    public TMappingExpression MaxDepth(int depth)
    {
        TypeMapActions.Add(tm => tm.MaxDepth = depth);

        return this as TMappingExpression;
    }
    public TMappingExpression ConstructUsingServiceLocator()
    {
        TypeMapActions.Add(tm => tm.ConstructUsingServiceLocator());

        return this as TMappingExpression;
    }
    public TMappingExpression BeforeMap(Action<TSource, TDestination> beforeFunction) => BeforeMapCore((src, dest, ctxt) => beforeFunction(src, dest));
    private TMappingExpression BeforeMapCore(Expression<Action<TSource, TDestination, ResolutionContext>> expr)

View on GitHub (pinned to dfa6dd587c)

Solutions

1. Pass a type that is derived from (a subtype of) the destination type.
2. If you do not need a type override, remove the As call entirely.
3. In generic code, add a generic constraint (where T : TDestination) and ensure T is actually more specific.

Example fix

// before — passing the same type
CreateMap<Animal, AnimalDto>().As(typeof(AnimalDto));

// after — passing a derived type, or remove As
CreateMap<Animal, AnimalDto>();
CreateMap<Dog, DogDto>().IncludeBase<Animal, AnimalDto>();

// Validate that the As type is derived from the destination before calling
Type asType = typeof(DerivedDest);
if (asType == typeof(TDestination))
    throw new ArgumentException($"As must specify a derived type, not {asType.Name} itself.");
if (!typeof(TDestination).IsAssignableFrom(asType))
    throw new ArgumentException($"{asType.Name} must derive from {typeof(TDestination).Name}.");

When it happens

Trigger: Calling .As(typeof(DestinationType)) or .As<TDestination>() where the type argument is identical to the map's destination type. This can happen in generic code where the type parameter resolves to the destination type at runtime.

Common situations: Generic helper code that resolves T to the destination type; copy-paste error; misunderstanding that As requires a strictly more derived type.

Related errors

• The type {typeMap.DestinationType.Name} does not have a cons
• Only member accesses are allowed. {destinationMember}
• Type '{conditionType.Name}' does not implement ICondition<TS
• Type '{conditionType.Name}' does not implement IPreCondition
• sourceExpression may not be null when mapping {DestinationMe

AI-assisted analysis of LuckyPennySoftware/AutoMapper@dfa6dd587c (2026-08-13). Data as JSON: /api/errors/1e45ffd32992604e. Report an issue: GitHub.