Mark strings as non-nullable in ASP.NET Core 3.0 Swagger

You can now use nullable reference types to mark strings as optional or required:

• Enable nullable reference types for your project/solution

• Mark strings in Dtos as nullable:

  public class ExampleDto
  {
     [StringLength(64, MinimumLength = 4)]
     public string RequiredString { get; set; }
  
     [StringLength(200)]
     public string? OptionalString { get; set; }
  }
  

• Enable Swagger support in your Startup.cs

  services.AddSwaggerGen(options =>
  {
      options.SupportNonNullableReferenceTypes();
  });
  

This leads to:

{
  "Example": {
    "type": "object",
    "properties": {
      "requiredString": {
        "maxLength": 64,
        "minLength": 4,
        "type": "string"
      },
      "optionalString": {
        "maxLength": 200,
        "minLength": 0,
        "type": "string",
        "nullable": true
      }
    },
    "additionalProperties": false
  }
}