Example of making HTTP Post and Get request from Android Kotlin to Web Api .NET Core

This example works only in Android Studio emulator and with accepting all certificates, and it is made with Retrofit library.

First example of Web Api in .NET Core:

using Microsoft.AspNetCore.Mvc;

namespace WebApi.Controllers;

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    [HttpGet]
    public string Get()
    {
        return "test";
    }

    [HttpGet("{id}")]
    public string Get(int id)
    {
        return "value";
    }

    [HttpPost]
    public string Post([FromBody] string value)
    {
        return $"Sent: {value}";
    }
}

Now example in Kotlin.

In \app\src\main\AndroidManifest.xml I have added internet permissions:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

In \app\src\main\res\layout\activity_main.xml I have added the button:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button"
        android:onClick="postHttp"
        tools:layout_editor_absoluteX="166dp"
        tools:layout_editor_absoluteY="441dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
        />

</androidx.constraintlayout.widget.ConstraintLayout>

In \app\build.gradle I have added

implementation "com.squareup.retrofit2:retrofit:2.9.0"
implementation "com.squareup.okhttp3:okhttp:4.9.0"
implementation 'com.squareup.retrofit2:converter-scalars:2.1.0'

Code to trust all certificates taken from here.

val trustAllCerts = arrayOf<TrustManager>(object : X509TrustManager {
    override fun checkClientTrusted(chain: Array<out X509Certificate>?, authType: String?) {
        Log.i(MainActivity::class.simpleName, "checkClientTrusted")
    }

    override fun checkServerTrusted(chain: Array<out X509Certificate>?, authType: String?) {
        Log.i(MainActivity::class.simpleName, "checkServerTrusted")
    }

    override fun getAcceptedIssuers() = arrayOf<X509Certificate>()
})
val sslContext = SSLContext.getInstance("SSL")
sslContext.init(null, trustAllCerts, java.security.SecureRandom())

// Create an ssl socket factory with our all-trusting manager
val sslSocketFactory = sslContext.socketFactory

// connect to server
val client = OkHttpClient.Builder().sslSocketFactory(sslSocketFactory, trustAllCerts[0] as X509TrustManager).hostnameVerifier{ _, _ -> true }.build()

Next building Retrofit:

val retro = Retrofit.Builder()
    .baseUrl("https://10.0.2.2:7037")
    .client(client)
    .addConverterFactory(ScalarsConverterFactory.create())
    .build()

Here notice baseUrl: https://10.0.2.2:7037 - 10.0.2.2 is localhost for Android Studio emulator

Add WebApiService interface:

interface WebApiService {
    @Headers("Content-Type: text/json")
    @POST("/api/Values")
    fun postMethod(@Body value: String): Call<String>
}

Create request:

val service = retro.create(WebApiService::class.java)
val webApiRequest = service.postMethod("\"test\"")

Here notice "\"test\"" since I am trying to send a raw string, otherwise would be rejected.

Last but not least, onResponse and onFailure:

val alertDialogBuilder = AlertDialog.Builder(this@MainActivity)

webApiRequest.enqueue(object : Callback<String> {
    override fun onResponse(call: Call<String>, response: Response<String>) {

        if (!response.isSuccessful) {
            alertDialogBuilder.setMessage(response.errorBody()!!.charStream().readText())
                .setCancelable(false)
                .setNeutralButton("OK", DialogInterface.OnClickListener { dialog, _ ->
                    dialog.dismiss()
                })

            val alert = alertDialogBuilder.create()
            alert.setTitle("Error")
            alert.show()
        }
        else {
            alertDialogBuilder.setMessage("Response: ${response.body().toString()}")
                .setCancelable(false)
                .setNeutralButton("OK", DialogInterface.OnClickListener { dialog, _ ->
                    dialog.dismiss()
                })

            val alert = alertDialogBuilder.create()
            alert.setTitle("Error")
            alert.show()
        }
    }

    override fun onFailure(call: Call<String>, t: Throwable) {
        alertDialogBuilder.setMessage(t.message)
            .setCancelable(false)
            .setNeutralButton("OK", DialogInterface.OnClickListener {
                    dialog, _ -> dialog.dismiss()
            })

        val alert = alertDialogBuilder.create()
        alert.setTitle("Error")
        alert.show()
    }

})

.NET solution download from here and Android source code download from here here.