Faced a error time of migration.

It looks like you're trying to add and update properties (`Code` and `By`) to an existing `Users` table in your database using Entity Framework Core migrations. There are a few things to check and correct in your migration and code snippet to ensure it works correctly:

### Migration Code

1. **Migration Configuration (`AddCodePropertiesToUserTable`):**
   - Ensure your migration class inherits from `Migration` or `MigrationBase` provided by Entity Framework Core.
   - Correct the table name in your migration operations (`"User"` vs `"Users"`).

   Here’s how you can correct and simplify your migration code:

   ```csharp
   public partial class AddCodePropertiesToUserTable : Migration
   {
       protected override void Up(MigrationBuilder migrationBuilder)
       {
           migrationBuilder.AddColumn(
               name: "Code",
               table: "Users", // Corrected table name
               type: "text",
               nullable: true);

           migrationBuilder.AddColumn(
               name: "By",
               table: "Users", // Corrected table name
               type: "text",
               nullable: true);

           migrationBuilder.CreateIndex(
               name: "IX_Code",
               table: "Users",
               column: "Code",
               unique: true); // Ensure this index is correct

           migrationBuilder.CreateIndex(
               name: "IX_By",
               table: "Users",
               column: "By");
       }

       protected override void Down(MigrationBuilder migrationBuilder)
       {
           migrationBuilder.DropIndex(
               name: "IX_Code",
               table: "Users");

           migrationBuilder.DropIndex(
               name: "IX_By",
               table: "Users");

           migrationBuilder.DropColumn(
               name: "Code",
               table: "Users");

           migrationBuilder.DropColumn(
               name: "By",
               table: "Users");
       }
   }
   ```

2. **Ensure Index Naming:**
   - Ensure that index names (`IX_Code`, `IX_By`) correspond correctly to the columns they are indexing.

### Code to Update Existing Users

1. **DbContext and Dependency Injection:**
   - Ensure `_dbContext` is correctly injected into your migration class or wherever `UpdateReferralValuesForExistingUser` method is located.

   ```csharp
   private readonly DbContext _dbContext;

   public AddCodePropertiesToUserTable(DbContext dbContext)
   {
       _dbContext = dbContext;
   }
   ```

2. **Update Existing Users Method:**
   - Call `UpdateReferralValuesForExistingUser` method in your migration to update existing users with generated codes.

   ```csharp
   protected override void Up(MigrationBuilder migrationBuilder)
   {
       // Add columns and indices as shown above

       // Update existing users with referral codes
       UpdateReferralValuesForExistingUser();
   }

   private void UpdateReferralValuesForExistingUser()
   {
       var usersWithoutReferralCode = _dbContext.Set()
           .Where(u => u.ReferralCode == null)
           .ToList();

       foreach (var user in usersWithoutReferralCode)
       {
           user.ReferralCode = GenerateUniqueReferralCode(8);
       }

       _dbContext.SaveChanges();
   }
   ```

### GenerateUniqueReferralCode Method

Ensure your `GenerateUniqueReferralCode` method is correctly generating unique referral codes:

```csharp
public string GenerateUniqueReferralCode(int length)
{
    if (length < 1)
    {
        throw new ArgumentException("Length must be greater than 0");
    }

    Guid guid = Guid.NewGuid();
    string base36String = NumberUtilities.ConvertToBase36(guid);

    if (base36String.Length >= length)
    {
        return base36String.Substring(0, length).ToUpper();
    }
    else
    {
        Random random = new Random();
        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

        while (base36String.Length < length)
        {
            base36String += chars[random.Next(chars.Length)];
        }

        return base36String.ToUpper();
    }
}
```

### Notes

- **Dependency Injection**: Ensure that `_dbContext` is correctly injected into your migration or class where the update method resides.
- **Migration Execution**: After correcting the migration code and ensuring `_dbContext` is properly set up, run `dotnet ef database update` or use the appropriate tooling to apply the migration to your database.

By following these steps, you should be able to add the `Code` and `By` properties to your `Users` table and update existing users with generated codes successfully. Adjust the specifics based on your actual DbContext and entity configurations as needed.