Why Does Entity Framework Throw an 'Cannot insert explicit value for identity column...' Error, and How Can I Fix It?

Entity Framework Error: Identity Columns and Explicit Value Insertion

One common issue encountered in Entity Framework (EF) is the inability to insert explicit values for identity columns with the error message: "Cannot insert explicit value for identity column in table..." when IDENTITY_INSERT is set to OFF in the database. This issue arises when EF attempts to manually insert a value into a column that is designated as an identity column, which automatically increments its values based on the database's settings.

Root Cause

The root cause of this error is that the database is responsible for assigning unique values to identity columns. When the StoreGeneratedPattern property of the entity's property is set to Identity, EF assumes that the database will handle the value assignment. However, if the code explicitly assigns a value to the ID column, a conflict occurs between the manual insertion and the database's automatic value generation logic, resulting in the error.

Solution

To resolve this issue, it is necessary to ensure that EF does not attempt to explicitly assign values to identity columns. This can be achieved by:

• Updating the EDMX File: Update the Entity Data Model (EDMX) file to reflect the database's configuration. Verify that the IsDbGenerated attribute for the identity column in the EF designer file is set to True. If it is not set, add it manually.
• Removing Explicit Value Assignment: Remove any explicit value assignment code from the entity's properties. EF will automatically handle the value generation for identity columns based on the database's settings.

Example

In the provided code example, the issue is likely caused by the explicit assignment of a value to the Id property of the GroupMember entity:

groupMember.Id = _groupContext.GroupMembers.Count();

To fix this, remove the explicit value assignment and allow EF to handle the value generation:

GroupMember groupMember = new GroupMember();
groupMember.GroupId = group.Id;
groupMember.UserId = (new UserId(group.Owner));
group.GroupMembers.Add(groupMember);

_groupContext.SaveChanges();

Conclusion

By adhering to these guidelines, developers can avoid the "Cannot insert explicit value for identity column..." error in Entity Framework, ensuring that identity columns are correctly managed by the database's automatic value generation logic.

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