Kotlin coroutine unit test fails with "Module with the Main dispatcher had failed to initialize"

Issue

While running unit test for kotlin suspend method which uses withContext(Dispatchers.Main) the test method fails with below exception:

My coroutine lib versions are kotlinx-coroutines-core:1.1.1 and kotlinx-coroutines-android:1.1.1

Example:

suspend fun methodToTest() {
        withContext(Dispatchers.Main) {
           doSomethingOnMainThread()
                val data = withContext(Dispatchers.IO) {
                    doSomethingOnIOThread()
                }
        }
    }

Also, when I remove the withContext(Dispatchers.Main) it works fine.

java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used

at kotlinx.coroutines.internal.MissingMainCoroutineDispatcher.missing(MainDispatchers.kt:79)
at kotlinx.coroutines.internal.MissingMainCoroutineDispatcher.isDispatchNeeded(MainDispatchers.kt:54)
at kotlinx.coroutines.DispatchedKt.resumeCancellable(Dispatched.kt:373)
at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:25)
at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:152)
at kotlinx.coroutines.BuildersKt.withContext(Unknown Source)

Solution

You don't have access to Dispatchers.Main in unit testing

See https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/

Dispatchers.Main Delegation part explains in detail what you need to do.

Answered By - r2rek