# Dependency

> The ECS-related data dependencies of the system.

## Definition

* **Type:** Property
* **Namespace:** [Unity.Entities](/engine/7000.0/script-reference/unity/entities.md)
* **Assembly:** UnityEngine.EntitiesModule

```csharp
protected JobHandle Dependency { get; set; }
```

### Remarks

Before [SystemBase.OnUpdate](/engine/7000.0/script-reference/unity/entities/systembase/onupdate.md), the Dependency property represents the combined job handles of any job that writes to the same components that the current system reads -- or reads the same components that the current system writes to. When you schedule \[IJobEntity] execution, the system uses the Dependency property to specify a job’s dependencies when scheduling it. The system also combines the new job's [JobHandle](/engine/7000.0/script-reference/unity/jobs/jobhandle.md) with Dependency so that any subsequent job scheduled in the system depends on the earlier jobs (in sequence).

The following example illustrates an `OnUpdate()` implementation that relies on implicit dependency management. The function schedules multiple IJobEntity jobs, each depending on the previous one:

You can opt out of this default dependency management by explicitly passing a [JobHandle](/engine/7000.0/script-reference/unity/jobs/jobhandle.md) when scheduling an IJobEntity. When you pass in a handle, the scheduling API returns a new handle representing the input dependencies combined with the new job. Handles from jobs scheduled with explicit dependencies are not automatically combined with the system’s Dependency property. You must set the Dependency property manually to propagate dependencies.

The following [SystemBase.OnUpdate](/engine/7000.0/script-reference/unity/entities/systembase/onupdate.md) function illustrates manual dependency management. The function schedules two IJobEntity jobs that do not depend upon each other (only the incoming system Dependency). Then a third job (IJobEntity or an [IJob](/engine/7000.0/script-reference/unity/jobs/ijob.md)) depends on both of the prior jobs; their handles are combined using [JobHandle.CombineDependencies](/engine/7000.0/script-reference/unity/jobs/jobhandle/combinedependencies.md). Finally, the resulting handle is assigned to the Dependency property so that the ECS safety manager can propagate the dependencies to subsequent systems.

You can combine implicit and explicit dependency management (by using [JobHandle.CombineDependencies]); however, doing so can be error prone. When you set the Dependency property, the assigned [JobHandle] replaces any existing dependency, it is not combined with them.

Note that the default, implicit dependency management does not include [IJobChunk](/engine/7000.0/script-reference/unity/entities/ijobchunk.md) jobs. You must manage the dependencies for [IJobChunk](/engine/7000.0/script-reference/unity/entities/ijobchunk.md) explicitly.

[JobHandle]: https://docs.unity3d.com/ScriptReference/Unity.Jobs.JobHandle.html

[JobHandle.CombineDependencies]: https://docs.unity3d.com/ScriptReference/Unity.Jobs.JobHandle.CombineDependencies.html