网上有好多解释，有些上升到了架构设计，让你觉得似乎非常高深莫测，特别是目前流行的ABP架构中就有提到多租户（IMustHaveTenant），其实说的简单一点就是再每一张数据库的表中添加一个TenantId的字段，用于区分属于不同的租户（或是说不同的用户组）的数据。关键是现实的方式必须对开发人员来说是透明的，不需要关注这个字段的信息，由后台或是封装在基类中实现数据的筛选和更新。

从新用户注册时就必须指定用户的TenantId,我的例子是用CompanyId,公司信息做为TenantId,哪些用户属于不同的公司，每个用户将来只能修改和查询属于本公司的数据。

接下来就是用户登录的时候获取用户信息的时候把TenantId保存起来，asp.net mvc（不是 core） 是通过 Identity 2.0实现的认证和授权，这里需要重写部分代码来实现。

最后用户对数据查询/修改/新增时把用户信息中TenantId，这里就需要设定一个Filter(过滤器)和每次SaveChange的插入TenantId

第一步，扩展 Asp.net Identity user 属性，必须新增一个TenantId字段,根据Asp.net Mvc 自带的项目模板修改IdentityModels.cs 这个文件

// You can add profile data for the user by adding more properties to your ApplicationUser class, please visit http://go.microsoft.com/fwlink/?LinkID=317594 to learn more.
public class ApplicationUser : IdentityUser
{
public async Task GenerateUserIdentityAsync(UserManager manager, string authenticationType)
{
// Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
var userIdentity = await manager.CreateIdentityAsync(this, authenticationType);
// Add custom user claims here
userIdentity.AddClaim(new Claim("http://schemas.microsoft.com/identity/claims/tenantid", this.TenantId.ToString()));
userIdentity.AddClaim(new Claim("CompanyName", this.CompanyName));
userIdentity.AddClaim(new Claim("EnabledChat", this.EnabledChat.ToString()));
userIdentity.AddClaim(new Claim("FullName", this.FullName));
userIdentity.AddClaim(new Claim("AvatarsX50", this.AvatarsX50));
userIdentity.AddClaim(new Claim("AvatarsX120", this.AvatarsX120));
return userIdentity;
}
public async Task GenerateUserIdentityAsync(UserManager manager)
{
// Note the authenticationType must match the one defined in CookieAuthenticationOptions.AuthenticationType
var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
// Add custom user claims here
return userIdentity;
}

 \[Display(Name = "全名")\]  
 public string FullName { get; set; }  
 \[Display(Name = "性别")\]  
 public int Gender { get; set; }  
 public int AccountType { get; set; }  
 \[Display(Name = "所属公司")\]  
 public string CompanyCode { get; set; }  
 \[Display(Name = "公司名称")\]  
 public string CompanyName { get; set; }  
 \[Display(Name = "是否在线")\]  
 public bool IsOnline { get; set; }  
 \[Display(Name = "是否开启聊天功能")\]  
 public bool EnabledChat { get; set; }  
 \[Display(Name = "小头像")\]  
 public string AvatarsX50 { get; set; }  
 \[Display(Name = "大头像")\]  
 public string AvatarsX120 { get; set; }  
 \[Display(Name = "租户ID")\]  
 public int TenantId { get; set; }  

}

public class ApplicationDbContext : IdentityDbContext
{
public ApplicationDbContext()
: base("DefaultConnection", throwIfV1Schema: false) => Database.SetInitializer(null);

 public static ApplicationDbContext Create() => new ApplicationDbContext();

}

　　第二步 修改注册用户的代码，注册新用户的时候需要选择所属的公司信息

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task Register(AccountRegistrationModel viewModel)
{
var data = this._companyService.Queryable().Select(x => new ListItem() { Value = x.Id.ToString(), Text = x.Name });
this.ViewBag.companylist = data;

   // Ensure we have a valid viewModel to work with  
   if (!this.ModelState.IsValid)  
   {  
     return this.View(viewModel);  
   }

   // Try to create a user with the given identity  
   try  
   {  
     // Prepare the identity with the provided information  
     var user = new ApplicationUser  
     {  
       UserName = viewModel.Username,  
       FullName = viewModel.Lastname + "." + viewModel.Firstname,  
       CompanyCode = viewModel.CompanyCode,  
       CompanyName = viewModel.CompanyName,  
       TenantId=viewModel.TenantId,  
       Email = viewModel.Email,  
       AccountType = 

     };  
     var result = await this.UserManager.CreateAsync(user, viewModel.Password);

     // If the user could not be created  
     if (!result.Succeeded)  
     {  
       // Add all errors to the page so they can be used to display what went wrong  
       this.AddErrors(result);

       return this.View(viewModel);  
     }

     // If the user was able to be created we can sign it in immediately  
     // Note: Consider using the email verification proces  
     await this.SignInAsync(user, true);

     return this.RedirectToLocal();  
   }  
   catch (DbEntityValidationException ex)  
   {  
     // Add all errors to the page so they can be used to display what went wrong  
     this.AddErrors(ex);

     return this.View(viewModel);  
   }  
 }

AccountController.cs

第三步 读取登录用户的TenantId 在用户查询和新增修改时把TenantId插入到表中,这里需要引用

Z.EntityFramework.Plus，这个是免费开源的一个类库，功能强大

public StoreContext()
: base("Name=DefaultConnection") {
//获取登录用户信息,tenantid
var claimsidentity = (ClaimsIdentity)HttpContext.Current.User.Identity;
var tenantclaim = claimsidentity?.FindFirst("http://schemas.microsoft.com/identity/claims/tenantid");
var tenantid = Convert.ToInt32(tenantclaim?.Value);
//设置当对Work对象进行查询时默认添加过滤条件
QueryFilterManager.Filter(q => q.Where(x => x.TenantId == tenantid));
//设置当对Order对象进行查询时默认添加过滤条件
QueryFilterManager.Filter(q => q.Where(x => x.TenantId == tenantid));
}

public override Task SaveChangesAsync(CancellationToken cancellationToken)
{
var currentDateTime = DateTime.Now;
var claimsidentity = (ClaimsIdentity)HttpContext.Current.User.Identity;
var tenantclaim = claimsidentity?.FindFirst("http://schemas.microsoft.com/identity/claims/tenantid");
var tenantid = Convert.ToInt32(tenantclaim?.Value);
foreach (var auditableEntity in this.ChangeTracker.Entries())
{
if (auditableEntity.State == EntityState.Added || auditableEntity.State == EntityState.Modified)
{
//auditableEntity.Entity.LastModifiedDate = currentDateTime;
switch (auditableEntity.State)
{
case EntityState.Added:
auditableEntity.Property("LastModifiedDate").IsModified = false;
auditableEntity.Property("LastModifiedBy").IsModified = false;
auditableEntity.Entity.CreatedDate = currentDateTime;
auditableEntity.Entity.CreatedBy = claimsidentity.Name;
auditableEntity.Entity.TenantId = tenantid;
break;
case EntityState.Modified:
auditableEntity.Property("CreatedDate").IsModified = false;
auditableEntity.Property("CreatedBy").IsModified = false;
auditableEntity.Entity.LastModifiedDate = currentDateTime;
auditableEntity.Entity.LastModifiedBy = claimsidentity.Name;
auditableEntity.Entity.TenantId = tenantid;
//if (auditableEntity.Property(p => p.Created).IsModified || auditableEntity.Property(p => p.CreatedBy).IsModified)
//{
// throw new DbEntityValidationException(string.Format("Attempt to change created audit trails on a modified {0}", auditableEntity.Entity.GetType().FullName));
//}
break;
}
}
}
return base.SaveChangesAsync(cancellationToken);
}

 public override int SaveChanges()  
 {  
   var currentDateTime = DateTime.Now;  
   var claimsidentity =(ClaimsIdentity)HttpContext.Current.User.Identity;  
   var tenantclaim = claimsidentity?.FindFirst("http://schemas.microsoft.com/identity/claims/tenantid");  
   var tenantid = Convert.ToInt32(tenantclaim?.Value);  
   foreach (var auditableEntity in this.ChangeTracker.Entries<Entity>())  
   {  
     if (auditableEntity.State == EntityState.Added || auditableEntity.State == EntityState.Modified)  
     {  
       auditableEntity.Entity.LastModifiedDate = currentDateTime;  
       switch (auditableEntity.State)  
       {  
         case EntityState.Added:  
           auditableEntity.Property("LastModifiedDate").IsModified = false;  
           auditableEntity.Property("LastModifiedBy").IsModified = false;  
           auditableEntity.Entity.CreatedDate = currentDateTime;  
           auditableEntity.Entity.CreatedBy = claimsidentity.Name;  
           auditableEntity.Entity.TenantId = tenantid;  
           break;  
         case EntityState.Modified:  
           auditableEntity.Property("CreatedDate").IsModified = false;  
           auditableEntity.Property("CreatedBy").IsModified = false;  
           auditableEntity.Entity.LastModifiedDate = currentDateTime;  
           auditableEntity.Entity.LastModifiedBy = claimsidentity.Name;  
           auditableEntity.Entity.TenantId = tenantid;  
           break;  
       }  
     }  
   }  
   return base.SaveChanges();  
 }

DbContext.cs

经过以上3步就实现一个简单的多租户查询数据的功能。

希望对大家有用。