https://docs.microsoft.com/en-us/ef/core/saving/cascade-delete#required-relationships
For optional relationships (nullable foreign key) it is possible to save a null foreign key value, which results in the following effects:
| Behavior Name | Effect on dependent/child in memory | Effect on dependent/child in database |
|---|---|---|
| Cascade | Entities are deleted | Entities are deleted |
| ClientSetNull (Default) | Foreign key properties are set to null | None |
| SetNull | Foreign key properties are set to null | Foreign key properties are set to null |
| Restrict | None | None |
For required relationships (non-nullable foreign key) it is not possible to save a null foreign key value, which results in the following effects:
| Behavior Name | Effect on dependent/child in memory | Effect on dependent/child in database |
|---|---|---|
| Cascade (Default) | Entities are deleted | Entities are deleted |
| ClientSetNull | SaveChanges throws | None |
| SetNull | SaveChanges throws | SaveChanges throws |
| Restrict | None | None |
原文：https://www.cnblogs.com/wswind/p/10723359.html