# Delegates should perform null-check when accessed

**URL:** <https://discuss.kotlinlang.org/t/delegates-should-perform-null-check-when-accessed/7853>\
**Category:** Language Design\
**Created:** [May 19, 2018, 3:41am UTC](https://discuss.kotlinlang.org/t/delegates-should-perform-null-check-when-accessed/7853 "2018-05-19T03:41:04Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![arocnies](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/arocnies/32/5143_2.png) [@arocnies](https://discuss.kotlinlang.org/u/arocnies)\
**Post date:** [May 19, 2018, 3:41am UTC](https://discuss.kotlinlang.org/t/delegates-should-perform-null-check-when-accessed/7853/1 "2018-05-19T03:41:04Z")

</div>

Kotlin property delegates do not infer types based on the property they’re associated with. For example:

```run-kotlin
import java.util.Scanner

fun methodThatReturnsAPlatformType() = Scanner("").findInLine("") // Returns type String!

fun main(args: Array<String>) {
//sampleStart
  val property: String by lazy { methodThatReturnsAPlatformType() } // Returns Lazy<String!>
  println(property) // Should throw IllegalStateException if `property` value is null
//sampleEnd
}

```

Now if we simply assigned the property instead of a Lazy delegate, we get the correct error:

```run-kotlin
import java.util.Scanner

fun methodThatReturnsAPlatformType() = Scanner("").findInLine("") // Returns type String!

fun main(args: Array<String>) {
//sampleStart
  val property: String = methodThatReturnsAPlatformType() // Returns String!
  println(property)
//sampleEnd
}

```

To work around this issue, we can explicitly set the type param for the delegate, `val property = lazy<String>`.

This [StackOverflow question](https://stackoverflow.com/questions/50173899/null-property-when-accessing-lazy-non-nullable-property-of-android-sensor) shows a case where @kolossal ran into this issue.

I believe this should be changed so that delegates returning a platform type should have an extra null check. All other cases where a non-nullable property is assigned a null value throws an error, so should delegates.

This example is in direct opposition to this paragraph from the Kotlin docs ([https://kotlinlang.org/docs/reference/java-interop.html#null-safety-and-platform-types](https://kotlinlang.org/docs/reference/java-interop.html#null-safety-and-platform-types)):

> If we choose a non-null type, the compiler will emit an assertion upon assignment. This prevents Kotlin’s non-null variables from holding nulls. Assertions are also emitted when we pass platform values to Kotlin functions expecting non-null values etc. Overall, the compiler does its best to prevent nulls from propagating far through the program (although sometimes this is impossible to eliminate entirely, because of generics).

---

<div class="post-metadata">

**Author:** ![ilya.gorbunov](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/ilya.gorbunov/32/1645_2.png) [@ilya.gorbunov](https://discuss.kotlinlang.org/u/ilya.gorbunov)\
**Post date:** [May 19, 2018, 4:12am UTC](https://discuss.kotlinlang.org/t/delegates-should-perform-null-check-when-accessed/7853/2 "2018-05-19T04:12:52Z")

</div>

This case is covered by the issue [https://youtrack.jetbrains.com/issue/KT-8135](https://youtrack.jetbrains.com/issue/KT-8135)