Best way to trim strings after data entry. Should

Question

I'm using ASP.NET MVC and I'd like all user entered string fields to be trimmed before they're inserted into the database. And since I have many data entry forms, I'm looking for an elegant way to trim all strings instead of explicitly trimming every user supplied string value. I'm interested to know how and when people are trimming strings.

I thought about perhaps creating a custom model binder and trimming any string values there...that way, all my trimming logic is contained in one place. Is this a good approach? Are there any code samples that do this?

Answer 1

There have been a lot of posts suggesting an attribute approach. Here is a package that already has a trim attribute and many others: Dado.ComponentModel.Mutations or NuGet

public partial class ApplicationUser
{
    [Trim, ToLower]
    public virtual string UserName { get; set; }
}

// Then to preform mutation
var user = new ApplicationUser() {
    UserName = "   M@X_speed.01! "
}

new MutationContext<ApplicationUser>(user).Mutate();

After the call to Mutate(), user.UserName will be mutated to m@x_speed.01!.

This example will trim whitespace and case the string to lowercase. It doesn't introduce validation, but the System.ComponentModel.Annotations can be used alongside Dado.ComponentModel.Mutations.

Answer 2

Extra info for anyone searching how to do this in ASP.NET Core 1.0. Logic has changed quite a lot.

I wrote a blog post about how to do it, it explains things in bit more detailed

So ASP.NET Core 1.0 solution:

Model binder to do the actual trimming

public class TrimmingModelBinder : ComplexTypeModelBinder  
{
    public TrimmingModelBinder(IDictionary propertyBinders) : base(propertyBinders)
    {
    }

    protected override void SetProperty(ModelBindingContext bindingContext, string modelName, ModelMetadata propertyMetadata, ModelBindingResult result)
    {
        if(result.Model is string)
        {
            string resultStr = (result.Model as string).Trim();
            result = ModelBindingResult.Success(resultStr);
        }

        base.SetProperty(bindingContext, modelName, propertyMetadata, result);
    }
}

Also you need Model Binder Provider in the latest version, this tells that should this binder be used for this model

public class TrimmingModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }

        if (context.Metadata.IsComplexType && !context.Metadata.IsCollectionType)
        {
            var propertyBinders = new Dictionary();
            foreach (var property in context.Metadata.Properties)
            {
                propertyBinders.Add(property, context.CreateBinder(property));
            }

            return new TrimmingModelBinder(propertyBinders);
        }

        return null;
    }
}

Then it has to be registered in Startup.cs

 services.AddMvc().AddMvcOptions(options => {  
       options.ModelBinderProviders.Insert(0, new TrimmingModelBinderProvider());
 });

Answer 3

Late to the party, but the following is a summary of adjustments required for MVC 5.2.3 if you are to handle the skipValidation requirement of the build-in value providers.

public class TrimStringModelBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // First check if request validation is required
        var shouldPerformRequestValidation = controllerContext.Controller.ValidateRequest && 
            bindingContext.ModelMetadata.RequestValidationEnabled;

        // determine if the value provider is IUnvalidatedValueProvider, if it is, pass in the 
        // flag to perform request validation (e.g. [AllowHtml] is set on the property)
        var unvalidatedProvider = bindingContext.ValueProvider as IUnvalidatedValueProvider;

        var valueProviderResult = unvalidatedProvider?.GetValue(bindingContext.ModelName, !shouldPerformRequestValidation) ??
            bindingContext.ValueProvider.GetValue(bindingContext.ModelName);

        return valueProviderResult?.AttemptedValue?.Trim();
    }
}

Global.asax

    protected void Application_Start()
    {
        ...
        ModelBinders.Binders.Add(typeof(string), new TrimStringModelBinder());
        ...
    }

Answer 4

  public class TrimModelBinder : DefaultModelBinder
  {
    protected override void SetProperty(ControllerContext controllerContext, 
      ModelBindingContext bindingContext, 
      System.ComponentModel.PropertyDescriptor propertyDescriptor, object value)
    {
      if (propertyDescriptor.PropertyType == typeof(string))
      {
        var stringValue = (string)value;
        if (!string.IsNullOrWhiteSpace(stringValue))
        {
          value = stringValue.Trim();
        }
        else
        {
          value = null;
        }
      }

      base.SetProperty(controllerContext, bindingContext, 
                          propertyDescriptor, value);
    }
  }

How about this code?

ModelBinders.Binders.DefaultBinder = new TrimModelBinder();

Set global.asax Application_Start event.

Answer 5

I disagree with the solution. You should override GetPropertyValue because the data for SetProperty could also be filled by the ModelState. To catch the raw data from the input elements write this:

 public class CustomModelBinder : System.Web.Mvc.DefaultModelBinder
{
    protected override object GetPropertyValue(System.Web.Mvc.ControllerContext controllerContext, System.Web.Mvc.ModelBindingContext bindingContext, System.ComponentModel.PropertyDescriptor propertyDescriptor, System.Web.Mvc.IModelBinder propertyBinder)
    {
        object value = base.GetPropertyValue(controllerContext, bindingContext, propertyDescriptor, propertyBinder);

        string retval = value as string;

        return string.IsNullOrWhiteSpace(retval)
                   ? value
                   : retval.Trim();
    }

}

Filter by propertyDescriptor PropertyType if you are really only interested in string values but it should not matter because everything what comes in is basically a string.

Answer 6

In ASP.Net Core 2 this worked for me. I'm using the [FromBody] attribute in my controllers and JSON input. To override the string handling in the JSON deserialization I registered my own JsonConverter:

services.AddMvcCore()
    .AddJsonOptions(options =>
        {
            options.SerializerSettings.Converters.Insert(0, new TrimmingStringConverter());
        })

And this is the converter:

public class TrimmingStringConverter : JsonConverter
{
    public override bool CanRead => true;
    public override bool CanWrite => false;

    public override bool CanConvert(Type objectType) => objectType == typeof(string);

    public override object ReadJson(JsonReader reader, Type objectType,
        object existingValue, JsonSerializer serializer)
    {
        if (reader.Value is string value)
        {
            return value.Trim();
        }

        return reader.Value;
    }

    public override void WriteJson(JsonWriter writer, object value,
        JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}