Why your React Native app can't connect to your local .NET API (And how to fix it)

Source: Dev.to

The Problem: What is “Localhost”?

When you type localhost inside your React Native code, the code is running on the mobile device (or emulator).

• On your computer, localhost refers to the computer itself.
• On an Android emulator, localhost refers to the Android device’s internal loopback network.

The emulator looks inside itself for a .NET server that doesn’t exist, resulting in a Network Error.

The Solution: The Magic IP Addresses

Testing on the iOS Simulator (Mac)

Apple’s iOS simulator shares the host machine’s network.

URL to use: http://localhost:/api/...

Testing on the Android Emulator

Android emulators run on an isolated virtual router. Google provides a special alias IP to reach the host machine.

URL to use: http://10.0.2.2:/api/...

Testing on a Physical Device (iPhone or Android via Wi‑Fi)

On a real phone, neither localhost nor 10.0.2.2 works. Use your computer’s local Wi‑Fi IP address.

1. Open a terminal and run ipconfig (Windows) or ifconfig (macOS).
2. Find your IPv4 address (e.g., 192.168.1.X or 10.0.0.X).

URL to use: http://192.168.1.X:/api/...

Note: Ensure your .NET API listens on all interfaces, not just localhost.
Example: dotnet run --urls "http://0.0.0.0:"

The Best Practice: Environment Configuration

Instead of manually changing Axios URLs for each platform, configure a dynamic environment variable.

.env file (Expo)

# Change this depending on your testing device!

# iOS Simulator
# EXPO_PUBLIC_API_URL=http://localhost:5257/api

# Android Emulator
EXPO_PUBLIC_API_URL=http://10.0.2.2:5257/api

# Physical Device
# EXPO_PUBLIC_API_URL=http://192.168.1.45:5257/api

Axios instance (TypeScript)

import axios from 'axios';

const api = axios.create({
  baseURL: process.env.EXPO_PUBLIC_API_URL,
  headers: {
    'Content-Type': 'application/json',
  },
});

export default api;

Now you only need to uncomment the appropriate line in .env, restart the Expo server with npx expo start -c (to clear the cache), and the API will connect correctly across all devices.