ASP.NET Web API Core: complex data model binder to trim strings

Question

Can someone shed some light of how to implement model binder for complex types? I'd like all string properties to be trimmed. I tried the following but "SetProperty" method is never called.

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);
    }
}

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 = context.Metadata.Properties.ToDictionary(p => p, context.CreateBinder);
            return new TrimmingModelBinder(propertyBinders);
        }

        return null;
    }
}

Found solution here: stackoverflow.com/questions/45495432/… — igorrider
– igorrider, Commented Nov 14, 2017 at 15:59

Johnny Oshika · Accepted Answer · 2018-05-07 23:09:10Z

4

Here's how we do it.

A simple helper extension method:

static class NormalizeString
{
    public static string TrimAndNullIfWhiteSpace(this string text) =>
       string.IsNullOrWhiteSpace(text)
       ? null
       : text.Trim();
}

Create a custom ModelBinder for strings:

public class StringModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        var modelName = bindingContext.ModelName;
        if (modelName == NullOrWhiteSpace)
            return Task.CompletedTask;

        var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);
        if(valueProviderResult == ValueProviderResult.None)
            return Task.CompletedTask;

        bindingContext.Result = ModelBindingResult.Success(
            valueProviderResult.FirstValue.TrimAndNullIfWhiteSpace());

        return Task.CompletedTask;
    }
}

Create a custom ModelBinderProvider that references the custom ModelBinder:

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

        if (context.Metadata.ModelType == typeof(string))
            return new BinderTypeModelBinder(typeof(StringModelBinder));

        return null;
    }
}

And finally, register the customer ModelBinderProvider in Startup's ConfigureServices:

    services
        .AddMvc(o =>
        {
            o.ModelBinderProviders.Insert(0, new ModelBinderProvider());
        })

answered May 7, 2018 at 23:09

Johnny Oshika

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2 Comments

Christopher King Over a year ago

I have implemented this and it seems to have the desired result so far, so thanks, but in your ModelBinderProvider class why not just return new StringModelBinder() instead of wrapping it in the BinderTypeModelBinder class; is there any advantage of doing so?

Johnny Oshika Over a year ago

@ChristopherKing It's been a few years, so I'm sorry I can't remember. What happens when you try without BinderTypeModelBinder?

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