How to set default query string parameter value for webapi?

Question

May I know how to set a default query string value for webapi in .NET 6?

Example code:
[HttpGet]
public Greeting Greeting(string name)
{
return $"Hello {name}!";
}

The name parameter needs to have a default value, if I use string name = "world", it seems not working and throw
"errors":{"name":["The name field is required."]}

Thanks in advanced.

Answer 1

Try the following pattern.

[HttpGet]  
public string Greeting(string name = "default value")  
{  
    return $"Hello {name}!";  
}  

You can also take a look at routing in ASP.NET Core

Routing in ASP.NET Core

Your sample code has an bug where you're returning a "Greeting" type but the actual return type is a string.

Answer 2

Hi @San Me ,

It is ok if the name is with a value, how about it you put in blank for the name? For my side, it get the error if name is blank although it has set to have a default value in parameter there.

I understand you. If the name is blank, it will show the 400 model invalid error:

In your scenario, to set the default value, you could try to use custom model binder. Refer to the following steps:

1. Create a custom model binder.

   //required reference: using Microsoft.AspNetCore.Mvc.ModelBinding;  
   // using Microsoft.AspNetCore.Mvc.ModelBinding.Binders;  
   public class CustomModelBinder : IModelBinder  
   {  
       public Task BindModelAsync(ModelBindingContext bindingContext)  
       {  
           if (bindingContext == null)  
           {  
               throw new ArgumentNullException(nameof(bindingContext));  
           }   
   
           var model = string.Empty;   
           if (bindingContext.ValueProvider.GetValue("name").FirstOrDefault() != null)  
           {  
               //if the parameter is not null. get the value.  
   

   pragma warning disable CS8604 // Possible null reference argument.

               model = bindingContext.ValueProvider.GetValue("name").FirstOrDefault();  
   

   pragma warning restore CS8604 // Possible null reference argument.

           }  
           else  
           {   
               //set the default value.  
               model = "Default Value";  
           }  
   
           bindingContext.Result = ModelBindingResult.Success(model);  
           return Task.CompletedTask;  
       }  
   }  
   
   public class MyCustomBinderProvider : IModelBinderProvider  
   {  
       public IModelBinder GetBinder(ModelBinderProviderContext context)  
       {  
           if (context == null)  
           {  
               throw new ArgumentNullException(nameof(context));  
           }  
   
           // specify the parameter your binder operates on  
           if (context.Metadata.ParameterName == "param2")  
           {  
               return new BinderTypeModelBinder(typeof(CustomModelBinder));  
           }  
   
           return null;  
       }  
   }  
   

2. Register the custom binder provider.

   builder.Services.AddControllers(opt => {  
       opt.ModelBinderProviders.Insert(0, new MyCustomBinderProvider());  
   });  
   

3. Applying ModelBinding Attribute on Action method

   // GET: api/<ToDoController>  
   [HttpGet]  
   public string Get([ModelBinder(BinderType = typeof(CustomModelBinder))]string name)  
   {  
       //return new string[] { "value1", "value2" };  
       return $"Hello {name}!";  
   }  
   

Then, the result is like this:

---

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Best regards,
Dillion