Explicit mappings

[xnoreq]

Is your feature request related to a problem? Please describe.
I see no way to prevent Mapperly from automatically generating mapping methods for nested properties and instead producing an error for a missing mapping.

Describe the solution you'd like
I'd like to see an ExplicitMappingRequired = true option in MapperAttribute which prevents this.

Example:

[Mapper]
public static partial class Mapper
{
    public static partial ATarget AToTarget(A a);
}

public class A
{
    public B B { get; set; }

    public A(B b)
    {
        B = b;
    }
}

public class B
{
}

public class ATarget
{
    public BTarget? B { get; set; }
}

public class BTarget
{
}

Currently generated code:

public static partial class Mapper
{
    [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "3.4.0.0")]
    public static partial global::ConsoleApp1.ATarget AToTarget(global::ConsoleApp1.A a)
    {
        var target = new global::ConsoleApp1.ATarget();
        target.B = MapToBTarget(a.B);
        return target;
    }

    [global::System.CodeDom.Compiler.GeneratedCode("Riok.Mapperly", "3.4.0.0")]
    private static global::ConsoleApp1.BTarget MapToBTarget(global::ConsoleApp1.B source)
    {
        var target = new global::ConsoleApp1.BTarget();
        return target;
    }
}

Suggested change:

[Mapper(RequireExplicitMapping = true)]
public static partial class Mapper
{
    public static partial ATarget AToTarget(A a);
}

-> Mapperly fails with an error during compilation because B cannot be mapped to BTarget.

Describe alternatives you've considered
Other mappers like Mapster or AutoMapper which support this feature.

[latonz]

Ah my bad, I understand you now, I also just learned we don't provide a conversion for new instance mappings.
What you are trying to achieve is not supported by Mapperly yet...

Idea: Never build a conversion except there is a user-method requesting exactly that mapping.

I'd be happy to accept a PR 😊
Hints (see also contributing docs):

• add a new configuration to the MapperAttribute
• If this is enabled: Returnnull when trying to build a mapping without a UserSymbol (e.g. in MappingBuilderContext.BuildMapping)