Extension methods that help convert Fluent Validation errors into MVC Model State errors.

Model State Collection.cs

public static class ModelStateCollectionExt
{
    public static void Update(this ModelStateDictionary modelState, ValidationResult result, bool usePropertyNames = false)
    {
        if (result.IsValid)
            return;

        foreach (var error in result.Errors)
        {
            // If we exclude the property name the error will show up in the validation summary
            var propertyName = usePropertyNames ? error.PropertyName : "";
            modelState.AddModelError(propertyName, error.ErrorMessage);
        }
    }

    public static void Update(this ModelStateDictionary modelState, DbEntityValidationException ex)
    {
        foreach (var property in ex.EntityValidationErrors)
        {
            foreach (var error in property.ValidationErrors)
            {
                // Exclude property name so that errors show up in the validation summary
                modelState.AddModelError("", error.ErrorMessage);
            }
        }
    }

    public static void AddModelError<TModel>(this ModelStateDictionary modelState, Expression<Func<TModel, object>> expr, string error)
    {
        var propertyName = ExpressionHelper.GetExpressionText(expr);
        modelState.AddModelError(propertyName, error);
    }
}

What namespace do I import or library do I reference for the ValidationResult type? I ask because I tried System.ComponentModel.DataAnnotations, but IsValid is not a member of ValidationResult.

you have to install the FluentValidation NuGet Package:

install-package FluentValidation

than use this namespace:

using FluentValidation.Results;