要实现在ASP.NET 4.5 Web API 2.0中使用JWT消息处理程序，并在Angular 7的HTTP拦截器中返回状态0，可以按照以下步骤进行操作：

1. 在ASP.NET Web API项目中安装以下NuGet包：

   • Microsoft.Owin.Security.Jwt
   • Microsoft.AspNet.WebApi.Owin

2. 在WebApiConfig.cs文件中注册JWT授权：

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        // 注册JWT授权
        ConfigureOAuth(config);

        // Web API 配置和服务
        config.MapHttpAttributeRoutes();

        // ...
    }

    private static void ConfigureOAuth(HttpConfiguration config)
    {
        var issuer = "your_issuer"; // JWT签发者
        var audience = "your_audience"; // JWT受众
        var secret = "your_secret"; // JWT密钥

        var tokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = issuer,
            ValidAudience = audience,
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(secret))
        };

        // 使用JWT授权
        config.SuppressDefaultHostAuthentication();
        config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

        // 配置JWT授权
        app.UseJwtBearerAuthentication(new JwtBearerAuthenticationOptions
        {
            AuthenticationMode = AuthenticationMode.Active,
            TokenValidationParameters = tokenValidationParameters
        });
    }
}

3. 创建一个名为JwtInterceptor的Angular 7 HTTP拦截器：

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent, HttpHeaders } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class JwtInterceptor implements HttpInterceptor {
    intercept(request: HttpRequest, next: HttpHandler): Observable> {
        // 获取JWT令牌
        const token = localStorage.getItem('jwtToken');

        // 添加JWT令牌到请求头
        if (token) {
            const headers = new HttpHeaders({
                'Authorization': `Bearer ${token}`
            });

            request = request.clone({ headers });
        }

        // 返回状态0
        return next.handle(request)
            .pipe(
                map((event: HttpEvent) => {
                    if (event.status === 0) {
                        // 处理状态0
                    }

                    return event;
                })
            );
    }
}

4. 在app.module.ts文件中注册JwtInterceptor：

import { NgModule } from '@angular/core';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';
import { JwtInterceptor } from './jwt.interceptor';

@NgModule({
    imports: [
        HttpClientModule
    ],
    providers: [
        { provide: HTTP_INTERCEPTORS, useClass: JwtInterceptor, multi: true }
    ]
})
export class AppModule { }

现在，当发送HTTP请求时，JwtInterceptor将在请求头中添加JWT令牌，并在状态为0时进行处理。