FluentValidation Extensions for AutoMapper 7 and 8

When the normal MVC validation is defined on a ViewModel and additional validation is done in the business-layer on the dto's using FluentValidation, the error messages are using the property names from the dto instead of the view-model. The following code shows this problem:

The flattened Person view model class.

public class Person
{
    [Required]
    public string FirstName { get; set; }

    public string Street { get; set; }

    public string City { get; set; }
}

The PersonDto model class:

public class PersonDto
{
    public string First { get; set; }

    public AddressDto Address { get; set; }
}

The AddressDto model class:

public class AddressDto
{
    public string Street { get; set; }

    public string City { get; set; }
}

When posting an invalid Person object to the WebAPI, you get an error back like (notice that the property names from the DTO are used!):

{
    "First": [
        "no 'a' allowed"
    ],
    "Address.City": [
        "The length of 'City' must be 2 characters or fewer. You entered 3 characters."
    ],
    "Address.Street": [
        "The length of 'Street' must be 3 characters or fewer. You entered 4 characters."
    ]
}

This library solves this issue by implementing a custom FluentValidation - PropertyNameResolver which uses the AutoMapper mapping definitions to translate the errors with DTO property names into view model errors. Example response will be like:

{
    "City": [
        "The length of 'City' must be 2 characters or fewer. You entered 3 characters."
    ],
    "Street": [
        "The length of 'Street' must be 3 characters or fewer. You entered 4 characters."
    ],
    "FirstName": [
        "no 'a' allowed"
    ]
}

To get this working you need the following changes for a DotNet Core MVC WebAPI project:

dotnet add package FluentValidationExtensions.AutoMapper

Let the DI framework inject an instance of IMapper into your Configure(...) method. You can then use the injected IMapper to create a new instance from the FluentValidationPropertyNameResolver and assign this to the static ValidatorOptions class.

-public void Configure(IApplicationBuilder app, IHostingEnvironment env)
+public void Configure(IApplicationBuilder app, IHostingEnvironment env, IMapper mapper)
{
     mapper.ConfigurationProvider.AssertConfigurationIsValid();

+    var resolver = new FluentValidationPropertyNameResolver(mapper);
+    ValidatorOptions.PropertyNameResolver = resolver.Resolve;
}

For a complete example, see the code from FluentValidationExample.Web.