使用ASP.Net Core Identity和JwtBearer身份验证方案将声明映射到上下文用户对象，可以按照以下步骤进行：

1. 首先，确保已经在项目中安装了Microsoft.AspNetCore.Identity和Microsoft.AspNetCore.Authentication.JwtBearer NuGet包。

2. 在Startup.cs文件中的ConfigureServices方法中配置身份验证服务：

services.AddIdentity()
    .AddEntityFrameworkStores()
    .AddDefaultTokenProviders();

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = "your-issuer",
            ValidAudience = "your-audience",
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("your-secret-key"))
        };
    });

3. 在Configure方法中启用身份验证中间件：

app.UseAuthentication();

4. 创建一个自定义的ClaimsTransformer类，用于将声明映射到上下文用户对象：

public class CustomClaimsTransformer : IClaimsTransformer
{
    private readonly UserManager _userManager;

    public CustomClaimsTransformer(UserManager userManager)
    {
        _userManager = userManager;
    }

    public async Task TransformAsync(ClaimsPrincipal principal)
    {
        var identity = (ClaimsIdentity)principal.Identity;

        if (identity.IsAuthenticated)
        {
            var user = await _userManager.GetUserAsync(principal);

            if (user != null)
            {
                // 将声明映射到上下文用户对象
                identity.AddClaim(new Claim("UserId", user.Id));
                identity.AddClaim(new Claim("Email", user.Email));
                // 添加其他需要的声明
            }
        }

        return principal;
    }
}

5. 在Startup.cs文件的ConfigureServices方法中注册CustomClaimsTransformer类：

services.AddTransient();

6. 更新JwtBearerOptions配置，以使用自定义的ClaimsTransformer：

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = "your-issuer",
            ValidAudience = "your-audience",
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("your-secret-key"))
        };

        options.Events = new JwtBearerEvents
        {
            OnTokenValidated = async context =>
            {
                var claimsTransformer = context.HttpContext.RequestServices.GetRequiredService();
                context.Principal = await claimsTransformer.TransformAsync(context.Principal);
            }
        };
    });

现在，当用户通过JwtBearer身份验证方案进行身份验证时，声明将自动映射到上下文用户对象中。您可以在控制器或其他部分的代码中访问这些声明，例如：

[Authorize]
[ApiController]
public class UserController : ControllerBase
{
    private readonly UserManager _userManager;
    
    public UserController(UserManager userManager)
    {
        _userManager = userManager;
    }
    
    [HttpGet]
    public async Task GetUserInfo()
    {
        var userId = User.FindFirst("UserId").Value;
        var user = await _userManager.FindByIdAsync(userId);
        
        // 使用用户对象进行操作
        // ...
        
        return Ok();
    }
}

这样，您就可以使用ASP.Net Core Identity和JwtBearer身份验证方案将声明映射到上下文用户对象了。