Best way to trim strings after data entry. Should I create a custom model binder?

One improvement to @takepara answer.

Somewere in project:

public class NoTrimAttribute : Attribute { }

In TrimModelBinder class change

if (propertyDescriptor.PropertyType == typeof(string))

to

if (propertyDescriptor.PropertyType == typeof(string) && !propertyDescriptor.Attributes.Cast<object>().Any(a => a.GetType() == typeof(NoTrimAttribute)))

and you can mark properties to be excluded from trimming with [NoTrim] attribute.

This is @takepara same resolution but as an IModelBinder instead of DefaultModelBinder so that adding the modelbinder in global.asax is through

ModelBinders.Binders.Add(typeof(string),new TrimModelBinder());

The class:

public class TrimModelBinder : IModelBinder
{
    public object BindModel(ControllerContext controllerContext,
    ModelBindingContext bindingContext)
    {
        ValueProviderResult valueResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        if (valueResult== null || valueResult.AttemptedValue==null)
           return null;
        else if (valueResult.AttemptedValue == string.Empty)
           return string.Empty;
        return valueResult.AttemptedValue.Trim();
    }
}

based on @haacked post: http://haacked.com/archive/2011/03/19/fixing-binding-to-decimals.aspx

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();
    }
}

  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.