How to access "Activity.this" in Kotlin?

Try this label instead

this@YourActivityName

You can get a reference to your MainActivity object in Kotlin by using a qualified this. e.g.:

class MyActivity : MainActivity() {
    val builder = MaterialDialog.Builder(this@MyActivity)
}

Answer is: this@ActivityName

For example: You should use it if you would like to define "Context" in MainActivity.kt

var mContext:Context = this@MainActivity

Why? Because in Kotlin language @ has mean "of" such as:

val a = this@A // A's this

If you want to learn more information, you can look Kotlin Language website: This Expression in Kotlin

If you are calling Activity.this from an inner class, you have to put inner before the class

class MyActivity : MainActivity() {
    // Call from class itself
    val builder = MaterialDialog.Builder(this@MyActivity) 

    inner class Inner {
        this@MyActivity // Call from the inner class 
    }
}