Error with Fluent validation using Generic

Question

I am trying to create a generic class for fluent validation. Refer the following code.

public abstract class GenericValidator<T> : AbstractValidator<T>
{
    /// <summary>
    /// Validates a string property to ensure it is not null, not empty, and meets a minimum length requirement.
    /// </summary>
    /// <param name="propertyValue">The string value to validate.</param>
    /// <param name="propertyName">The name of the property being validated.</param>
    /// <param name="minLength">The minimum length the string must have.</param>
    /// <param name="errorMessage">The error message format.</param>
    /// <returns>Returns a rule builder for further customization.</returns>
    protected IRuleBuilderOptions<T, string> ValidateString(
        Func<T, string> propertyValue,
        string propertyName,
        int minLength = 0,
        string errorMessage = "{PropertyName} is invalid.")
    {
        return *RuleFor*(propertyValue)
            .NotNull().WithMessage($"{propertyName} cannot be null.")
            .NotEmpty().WithMessage($"{propertyName} cannot be empty.")
            .MinimumLength(minLength).WithMessage($"{propertyName} must be at least {minLength} characters long.")
            .WithMessage(errorMessage);
    }
}

I am getting the error

"CS0411 The type arguments for method 'AbstractValidator<T>.RuleFor<TProperty>(Expression<Func<T, TProperty>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly" for "RuleFor" method.

I have limited exposure with Generic. I tried to solve the same but could not. Any inputs would be helpful. I am using Fluent validation package with version 11* for this.

What needs to be done to solve the error?

Answer 1

According to your comment, the problem seems to be that RuleFor expects a System.Linq.Expressions.Expression<System.Func<T,string>> parameter, not a System.Func<T,string> parameter. Therefore, change the propertyValue parameter type in ValidateString:

protected IRuleBuilderOptions<T, string> ValidateString(
        Expression<Func<T, string>> propertyValue,
        string propertyName,
        int minLength = 0,
        string errorMessage = "{PropertyName} is invalid.")

Now, the error for RuleFor should disappear. If it doesn't, you can still specify the generic type parameter:

return RuleFor<string>(propertyValue)
    ...

The difference between Func<> and Expression<Func<>> is that the former specifies a delegate, i.e., a kind of function address pointing to an executable function, where as the latter specifies an expression tree describing an expression. RuleFor uses this expression tree to determine the property name from a compilable expression rather than from an arbitrary string value.

Make sure to pass the propertyName argument by using a nameof expression.