Solving [FromQuery] convert empty string to null issue

While working with ASP.NET Core, you may encounter a problem where a [FromQuery] parameter is converted to null when it is passed as an empty string. This may not be a problem most of the time, but if you want to implement functionalities like searching for names that contain spaces, it can become a problem. In this blog post, we'll discuss how to solve this problem using a custom ModelBinder.

To demonstrate this issue, we created an ASP.NET Core Web API project and a controller with 4 actions.

[ApiController]
[Route("[controller]")]
public class ModelBinderController : ControllerBase
{
    [HttpGet("simple")]
    public IActionResult GetSimpleType([FromQuery] string value)
    {
        return Ok($"GetSimpleType: Value is [{value}]");
    }
    [HttpGet("complex")]
    public IActionResult GetComplexType([FromQuery] Model model)
    {
        return Ok($"GetComplexType: Value is [{model.Value}]");
    }
    [HttpPost("simple")]
    public IActionResult PostSimpleType([FromBody] string value)
    {
        return Ok($"PostSimpleType: Value is [{value}]");
    }
    [HttpPost("complex")]
    public IActionResult PostComplexType([FromBody] Model model)
    {
        return Ok($"PostComplexType: Value is [{model.Value}]");
    }
}

public class Model
{
    public string Value { get; set; }
}

We will test both simple string or complex type that has a string value, also in get and post methods. Below is the result.

POST https://localhost:7132/modelbinder/simple
body: "value"
returns: PostSimpleType: Value is [m22odel]
body: "   "
returns: PostSimpleType: Value is [   ]

POST https://localhost:7132/modelbinder/complex
body { "value"; "   " }
returns: PostComplexType: Value is [  ]
body { "value": "value" }
returns: PostComplexType: Value is [value]

GET https://localhost:7132/modelbinder/simple?value=value
returns: GetSimpleType: Value is [value]
GET https://localhost:7132/modelbinder/simple?value=%20%20%20
returns: GetSimpleType: Value is [] # wrong

GET https://localhost:7132/modelbinder/complex?value=value
returns: GetComplexType: Value is [value]
GET https://localhost:7132/modelbinder/complex?value=%20%20%20
returns: GetComplexType: Value is [] # wrong

It looks like all post requests run correctly. But for get requests, the empty string %20%20%20 is converted to null.

The reason for this is in SimpleTypeModelBinder, where there is a property called ConvertEmptyStringToNull, and the default value is true. Therefore, white spaces are converted to null. To resolve this, we just need to set it to false.

{
    // Already have a string. No further conversion required but handle ConvertEmptyStringToNull.
    if (bindingContext.ModelMetadata.ConvertEmptyStringToNull && string.IsNullOrWhiteSpace(value))
    {
        model = null;
    }
    else
    {
        model = value;
    }
}

First, we need to create a custom ModelBinder class that inherits from the IModelBinder interface. In this class, we'll implement the BindModelAsync method. This method will be called by the framework to bind the model.

public class EnhancedSimpleTypeModelBinder : IModelBinder
{
    private SimpleTypeModelBinder baseBinder;
    public EnhancedSimpleTypeModelBinder(Type type, ILoggerFactory loggerFactory)
    {
        this.baseBinder = new SimpleTypeModelBinder(type, loggerFactory);
    }

    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        (bindingContext.ModelMetadata as DefaultModelMetadata).DisplayMetadata.ConvertEmptyStringToNull = false;
        return baseBinder.BindModelAsync(bindingContext);
    }
}

Then, we need to create the ModelBinderProvider class. This class will create instances of our custom ModelBinder.

public class EnhancedSimpleTypeModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (!context.Metadata.IsComplexType)
        {
            var loggerFactory = context.Services.GetRequiredService<ILoggerfactory>();
            return new EnhancedSimpleTypeModelBinder(context.Metadata.ModelType, loggerFactory);
        }
        return null;
    }
}

Finally, we need to register our custom ModelBinder in the application. We can do this by adding the following code to the Program.cs:

builder.Services.AddControllers(options =>
{
    var originProvider = options.ModelBinderProviders.First(p => p.GetType() == typeof(SimpleTypeModelBinderProvider));
    options.ModelBinderProviders[options.ModelBinderProviders.IndexOf(originProvider)] = new EnhancedSimpleTypeModelBinderProvider();
});

Now let’s test again.

POST https://localhost:7132/modelbinder/simple
body: "value"
returns: PostSimpleType: Value is [m22odel]
body: "   "
returns: PostSimpleType: Value is [   ]

POST https://localhost:7132/modelbinder/complex
body { "value"; "   " }
returns: PostComplexType: Value is [  ]
body { "value": "value" }
returns: PostComplexType: Value is [value]

GET https://localhost:7132/modelbinder/simple?value=value
returns: GetSimpleType: Value is [value]
GET https://localhost:7132/modelbinder/simple?value=%20%20%20
returns: GetSimpleType: Value is [   ] # correct

GET https://localhost:7132/modelbinder/complex?value=value
returns: GetComplexType: Value is [value]
GET https://localhost:7132/modelbinder/complex?value=%20%20%20
returns: GetComplexType: Value is [   ] # correct

Everything works fine!