dotnet/efcore · error · NotSupportedException

Parameter clash during expression lifting for: {parameter.Na

Error message

Parameter clash during expression lifting for: {parameter.Name}

What it means

Thrown in VisitBlock (expression context) when _liftedState.Variables.TryAdd(parameter, uniquifiedName) fails - the same ParameterExpression instance is already registered in the lifted variables for the current lift scope. NotSupportedException. Part of EF Core's LINQ-to-C# syntax translation.

Source

Thrown at src/EFCore.Design/Query/Internal/LinqToCSharpSyntaxTranslator.cs:507

        var stackFrame = _stack.Peek();

        // Do a 1st pass to identify and register any labels, since goto can appear before its label.
        PreprocessLabels();

        // Go over the block's variables, assign names to any unnamed ones and uniquify. Then add them to our stack frame, unless
        // this is an expression block that will get lifted.
        foreach (var parameter in block.Variables)
        {
            var (variables, variableNames) = (stackFrame.Variables, stackFrame.VariableNames);

            var uniquifiedName = UniquifyVariableName(parameter.Name ?? "unnamed");

            if (blockContext == ExpressionContext.Expression)
            {
                if (!_liftedState.Variables.TryAdd(parameter, uniquifiedName))
                {
                    throw new NotSupportedException("Parameter clash during expression lifting for: " + parameter.Name);
                }

                _liftedState.VariableNames.Add(uniquifiedName);
            }
            else
            {
                if (!variables.TryAdd(parameter, uniquifiedName))
                {
                    throw new InvalidOperationException(
                        DesignStrings.SameParameterExpressionDeclaredAsVariableInNestedBlocks(parameter.Name ?? "<null>"));
                }

                variableNames.Add(uniquifiedName);
            }
        }

        var unassignedVariables = block.Variables.ToList();

View on GitHub (pinned to dbf9771522)

Solutions

1. Use distinct ParameterExpression instances for each variable scope being lifted.
2. Rebuild the expression tree with fresh parameters instead of reusing one.

Example fix

// before
var p = Expression.Parameter(typeof(int), "x");
// same 'p' reused in two lifted blocks
// after
var p1 = Expression.Parameter(typeof(int), "x");
var p2 = Expression.Parameter(typeof(int), "x");

// Ensure ParameterExpression instances are unique within a lifted scope
var seen = new HashSet<ParameterExpression>();
foreach (var block in AllBlocks(expr))
    foreach (var v in block.Variables)
        if (!seen.Add(v))
            throw new InvalidOperationException(
                $"ParameterExpression '{v.Name}' is reused across lifted blocks; create distinct instances.");

When it happens

Trigger: An expression block being lifted that declares a parameter already present in the lifted state (same ParameterExpression instance reused across lifted blocks).

Common situations: Programmatically reusing the same ParameterExpression instance across blocks that get lifted together; expression-tree construction bugs sharing a parameter variable.

Related errors

• Lifted expressions remaining at top-level in expression cont
• The same ParameterExpression instance with name '{parameter}
• Label on last expression of an expression block
• TranslateCatchBlock: unnamed parameter as catch variable
• Trying to evaluate a non-expression condition in expression

AI-assisted analysis of dotnet/efcore@dbf9771522 (2026-08-06). Data as JSON: /api/errors/43757c750b11d5c4. Report an issue: GitHub.