GraphQL 既是一种用于 API 的查询语言也是一个满足你数据查询的运行时。GraphQL 对你的 API 中的数据提供了一套易于理解的完整描述，使得客户端能够准确地获得它需要的数据，而且没有任何冗余，也让 API 更容易地随着时间推移而演进，还能用于构建强大的开发者工具。

                                        ——出自 https://graphql.cn

　　HotChocolate可以通过自定义Directive来对字段的值进行转换和处理，下在的例子就是给字符串给字符串类型的值转大写和替换。

using HotChocolate;
using HotChocolate.Data;
using HotChocolate.Execution;
using HotChocolate.Types;
using System;
using System.Collections.Generic;
using System.Reflection;

namespace GraphQLBase003
{
    class Program
    {
        static void Main(string[] args)
{
            DirectiveDemo.Run();
        }
    }
  
    public class DirectiveDemo
    {
        public static void Run()
{
            var schema = SchemaBuilder.New()
                .AddProjections()
                .AddQueryType<Query>()
                .AddDirectiveType<UpperDirectiveType>()
                .AddDirectiveType<ReplaceDirectiveType>()
                .Create();
            var executor = schema.MakeExecutable();
            Console.WriteLine("原name=abcde ");
            Console.WriteLine("--------------转大写-------------------");
            Console.WriteLine(executor.Execute("{ student{id name @upper(name:\"this is test\")  age} }").ToJson());
            Console.WriteLine("--------------a替换成1 -------------------");
            Console.WriteLine(executor.Execute("{ student{id name @replace(old:\"a\",new:\"1\")  age} }").ToJson());
            Console.WriteLine("--------------然后全部转大写-.a替换成1 -------------------");
            Console.WriteLine(executor.Execute("{ student{id name @upper(name:\"this is test\") @replace(old:\"a\",new:\"1\")  age} }").ToJson());
            Console.WriteLine("--------------a替换成1.然后全部转大写-------------------");
            Console.WriteLine(executor.Execute("{ student{id name @replace(old:\"a\",new:\"1\")  @upper(name:\"this is test\") age} }").ToJson());
        }
        public class Query
        {
            [UseProjection]
            public Student GetStudent()
{
                return new Student
                {
                    Id = 1,
                    Name = "abcde",
                    Age = 234
                };
            }
            [UseProjection]
            public List<Student> GetStudents()
            {
                return new List<Student>{
                    new Student
                    {
                        Id = 100,
                        Name = "aBcD",
                        Age=10
                    },
                    new Student
                    {
                        Id = 101,
                        Name = "EFGH",
                        Age=20
                    }
                };
            }
        }
        public class Student
        {
            public int Id { get; set; }
            public string Name { get; set; }
            public int Age { get; set; }
        }

        public class UpperDirectiveType : DirectiveType<UpperDirective>
        {
            protected override void Configure(IDirectiveTypeDescriptor<UpperDirective> descriptor)
{
                descriptor.Name("upper");
                descriptor.Location(DirectiveLocation.Field);
                descriptor.Use(next => context =>
                {
                    foreach (var directive in context.FieldSelection.Directives)
                    {
                        if (directive.Name.Value == "upper")
                        {
                            if (context.Field.Member.MemberType == System.Reflection.MemberTypes.Property)
                            {
                                var pro = context.Field.Member as PropertyInfo;
                                var obj = context.GetType().GetMethod("Parent").MakeGenericMethod(context.ObjectType.RuntimeType).Invoke(context, new object[0]);
                                var value = pro.GetValue(obj);
                                pro.SetValue(obj, value.ToString().ToUpper());                            
                            }
                        }
                    }
                    return next.Invoke(context);
                });
            }
        }

        public class UpperDirective
        {
            public string Name
            {
                get;
                set;
            }
        }

        public class ReplaceDirectiveType : DirectiveType<ReplaceDirective>
        {

            protected override void Configure(IDirectiveTypeDescriptor<ReplaceDirective> descriptor)
{
                descriptor.Name("replace");
                descriptor.Location(DirectiveLocation.Field);
                descriptor.Use(next => context =>
                {
                    foreach (var directive in context.FieldSelection.Directives)
                    {
                        if (directive.Name.Value == "replace")
                        {
                            var dir = new Dictionary<string, object>();
                            foreach (var item in directive.Arguments)
                            {
                                dir.Add(item.Name.Value?.ToLower(), item.Value.Value);
                            }
                            if (context.Field.Member.MemberType == System.Reflection.MemberTypes.Property)
                            {                                
                                var s = context.Parent<Student>();
                                var pro = context.Field.Member as PropertyInfo;
                                var obj = context.GetType().GetMethod("Parent").MakeGenericMethod(context.ObjectType.RuntimeType).Invoke(context, new object[0]);
                                var value = pro.GetValue(obj);
                                pro.SetValue(obj, value.ToString().Replace(dir["old"].ToString(), dir["new"].ToString()));                                
                            }
                        }
                    }
                    return next.Invoke(context);
                });


            }
        }

        public class ReplaceDirective
        {
            public string Old
            {
                get;
                set;
            }
            public string New
            {
                get;
                set;
            }
        }
    }
}

　　upper和replace两个Directive处理的还比较粗糙，这里主要说明Directive的定义方式；在调用这些Directive时，按照前后顺序调用，多个Directive可以同时生效，和asp.net core的中间件原理相近，上例的后两个调用 name @upper @replace和name @replace @upper返回的结果是不一样的。

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