dotnet/efcore · error · InvalidOperationException

The result column '{column}' cannot be added to the stored p

Error message

The result column '{column}' cannot be added to the stored procedure '{sproc}' because another result column with this name already exists.

What it means

Thrown when two result columns on the same stored procedure share the same store column name. The validator tracks names in a HashSet and rejects a duplicate add. Each result column must have a unique store name within a single sproc.

Source

Thrown at src/EFCore.Relational/Infrastructure/RelationalModelValidator.cs:666

                storeGeneratedProperties.Remove(property.Name);
            }
        }

        var resultColumnNames = new HashSet<string>();
        foreach (var resultColumn in sproc.ResultColumns)
        {
            IProperty? property = null!;
            if (resultColumn.PropertyName != null
                && !properties.TryGetValue(resultColumn.PropertyName, out property))
            {
                throw new InvalidOperationException(
                    RelationalStrings.StoredProcedureResultColumnNotFound(
                        resultColumn.PropertyName, entityType.DisplayName(), storeObjectIdentifier.DisplayName()));
            }

            if (!resultColumnNames.Add(resultColumn.Name))
            {
                throw new InvalidOperationException(
                    RelationalStrings.StoredProcedureDuplicateResultColumnName(
                        resultColumn.Name, storeObjectIdentifier.DisplayName()));
            }

            if (resultColumn.PropertyName == null)
            {
                continue;
            }

            switch (storeObjectIdentifier.StoreObjectType)
            {
                case StoreObjectType.InsertStoredProcedure:
                case StoreObjectType.UpdateStoredProcedure:
                    if (!storeGeneratedProperties.ContainsKey(property.Name))
                    {
                        throw new InvalidOperationException(
                            RelationalStrings.StoredProcedureResultColumnNotGenerated(
                                entityType.DisplayName(), resultColumn.PropertyName, storeObjectIdentifier.DisplayName()));

View on GitHub (pinned to dbf9771522)

Solutions

Remove the duplicate HasResultColumn call.
Rename one of the result columns so the store names differ.
Consolidate the two mappings into a single result column.

Example fix

// before
sp.HasResultColumn("Id");
sp.HasResultColumn("Id"); // duplicate

// after
sp.HasResultColumn("Id");

Defensive patterns

Strategy: validation

Validate before calling

foreach (var et in modelBuilder.Model.GetEntityTypes())
{
    foreach (var s in new[] { et.GetInsertStoredProcedure(), et.GetUpdateStoredProcedure(), et.GetDeleteStoredProcedure() }.Where(s => s != null))
    {
        var seen = new HashSet<string>();
        foreach (var rc in s.ResultColumns)
            if (!seen.Add(rc.Name)) throw new InvalidOperationException($"Duplicate result column {rc.Name} on {s.GetStoreIdentifier().Name}");
    }
}

Try / catch

try { _ = ctx.Model; } catch (InvalidOperationException ex) when (ex.Message.Contains("another result column with this name already exists")) { /* remove the duplicate result column */ }

Prevention

Avoid declaring the same result column twice; build result-column config from a single source.

When it happens

Trigger: Two .HasResultColumn("Id") calls on the same sproc; mapping the same store column name twice by mistake.

Common situations: Accidental duplicate config lines; merging two builders that both declare the same result column.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/cfd58646f6d315f9. Report an issue: GitHub.