What causes "MemberInit: incompatible MemberListBinding" in dotnet/efcore?

Use collection member types that implement IEnumerable with a single-argument Add.

dotnet/efcore · error · NotImplementedException

MemberInit: incompatible MemberListBinding

Error message

MemberInit: incompatible MemberListBinding

What it means

C# collection-initializer syntax requires an IEnumerable member with a single-argument Add method. If a MemberListBinding (collection member binding) does not fit — the member is not IEnumerable, or its Add method takes multiple arguments or is not named Add — the translator cannot lift those calls yet and throws NotImplementedException("MemberInit: incompatible MemberListBinding").

Source

Thrown at src/EFCore.Design/Query/Internal/LinqToCSharpSyntaxTranslator.cs:2557

            }

            var liftedStatementsPosition = _liftedState.Statements.Count;

            VisitMemberBinding(binding);

            initializerExpressions.Add((AssignmentExpressionSyntax)Result!);

            if (_liftedState.Statements.Count > liftedStatementsPosition)
            {
                // TODO: This is tricky because of the recursive nature of MemberMemberBinding
                throw new NotImplementedException("MemberInit: lifted statements");
            }
        }

        if (incompatibleListBindings is not null)
        {
            // TODO: Lift the instantiation and add extra statements to add the incompatible bindings after that
            throw new NotImplementedException("MemberInit: incompatible MemberListBinding");
        }

        Result = objectCreation.WithInitializer(
            InitializerExpression(
                SyntaxKind.ObjectInitializerExpression,
                SeparatedList<ExpressionSyntax>(initializerExpressions)));

        return memberInit;
    }

    /// <inheritdoc />
    protected override Expression VisitListInit(ListInitExpression listInit)
    {
        var objectCreation = Translate<ObjectCreationExpressionSyntax>(listInit.NewExpression);

        List<ElementInit>? incompatibleListBindings = null;

        var initializerExpressions = new List<ExpressionSyntax>(listInit.Initializers.Count);

View on GitHub (pinned to dbf9771522)

Solutions

Use collection member types that implement IEnumerable with a single-argument Add.
Initialize the collection after construction via explicit Add calls rather than a member list binding.
Avoid collection member bindings on custom-add collections in precompiled queries.

Example fix

// before
new X { Items = { a, b } }  // where Items is a custom collection with Add(int,string)
// after
var x = new X(); x.Items.Add(a); x.Items.Add(b); // explicit adds after construction

Defensive patterns

Strategy: validation

Validate before calling

// Flag collection member bindings whose member is not IEnumerable-with-single-arg-Add
protected override Expression VisitMemberInit(MemberInitExpression mi) {
    foreach (var b in mi.Bindings.OfType<MemberListBinding>()) {
        var t = b.Member.GetMemberType();
        if (!typeof(IEnumerable).IsAssignableFrom(t)
            || b.Initializers.Any(e => e.AddMethod.Name != "Add" || e.Arguments.Count != 1)) Found = true;
    }
    return mi;
}

Prevention

Use IEnumerable collections with a single-argument Add for member list bindings.
Initialize custom collections via explicit Add calls after construction.
Avoid multi-argument Add collections in member initializers.

When it happens

Trigger: An object initializer binding a collection member whose Add method has multiple parameters or is not named Add, or whose member type is not IEnumerable.

Common situations: Custom collection types with Add(int, string) signatures; non-IEnumerable members initialized with list bindings inside compiled expressions.

Related errors

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/d250ff5cb48f57db. Report an issue: GitHub.