String Trim Model Binder in ASP .NET Core 2

Question

I'm working on a .NET Core 2 API project and have been trying to implement a universal string trim model binder that would trim all string values of provided request parameters and field values. So far I have had mixed results and am struggling to find working example that would point me in the right direction. I've been trying to implement the same model binder as posted by Vikash Kumar.

This model binder works fine for all string values that are passed into controller actions via direct parameters, such as public IActionResult Profile(string username), but for string fields in complex objects the BindModelAsync method of the TrimmingModelBinder class never gets called. An example of an HttpPost action in my controller would be public IActionResult Profile([FormBody] ProfileLookupModel model). The model binder does not seem to check the fields of the complex model. It also doesn't work for fields that are Lists of strings.

I recall prior to .NET Core, specifying a string trim model binder would recursively check each field of complex models, even models within complex models. This doesn't seem to be the case in .NET Core, but I might be wrong. My project is targeting the netcoreapp2.0 framework.

I'm curious if anyone has had the same issue as me and possibly found a solution for it.

Note: I haven't posted any sample code as it is the same as the code from the referenced article.

Answer 1

I'll add my 2 cents here. Instead of using some kind of model binding hook, I went to a action filter. One of the advantages is that the developer can select which actions to use, instead of having this processing for all requests and model bindings (not that it should affect performance that much). Btw action filters can also be applied globally.

Here is my code, first create an action filter.

public class TrimInputStringsAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext context)
    {
        foreach (var arg in context.ActionArguments.ToList())
        {
            if (arg.Value is string)
            {
                if (arg.Value == null) 
                { 
                    continue; 
                }

                string val = arg.Value as string;
                if (!string.IsNullOrEmpty(val))
                {
                    context.ActionArguments[arg.Key] = val.Trim();
                }

                continue;
            }

            Type argType = arg.Value.GetType();
            if (!argType.IsClass)
            {
                continue;
            }

            TrimAllStringsInObject(arg.Value, argType);
        }
    }

    private void TrimAllStringsInObject(object arg, Type argType)
    {
        var stringProperties = argType.GetProperties()
                                      .Where(p => p.PropertyType == typeof(string));

        foreach (var stringProperty in stringProperties)
        {
            string currentValue = stringProperty.GetValue(arg, null) as string;
            if (!string.IsNullOrEmpty(currentValue))
            {
                stringProperty.SetValue(arg, currentValue.Trim(), null);
            }
        }
    }
}

To use it, either register as global filter or decorate your actions with the TrimInputStrings attribute.

[TrimInputStrings]
public IActionResult Register(RegisterViewModel registerModel)
{
    // Some business logic...
    return Ok();
}

Answer 2

TrimmingModelBinder is essentially configured for strings only, and defaults back to SimpleTypeModelBinder if it fails, or other binders configured. So if your implementation is essentially the same as in TrimmingModelBinder then it will definitely work for strings only.

For complex types, I recommend creating a new binder, and its corresponding provider, which will have to check all string properties in the model type and trim the value before binding. Then register this binder at index 0 such that its the first one checked before any other binders are tried.

services.AddMvc(options => option.ModelBinderProviders.Insert(0, new MyComplexTypeModelBinderProvider());