Repository: dotnet/efcore
File: src/EFCore/Query/Internal/ExpressionTreeFuncletizer.cs (C#)

Code before:
            {
                parameterName = tempParameterName ?? "p";

                var compilerPrefixIndex = parameterName.LastIndexOf('>');
                if (compilerPrefixIndex != -1)
                {
                    parameterName = parameterName[(compilerPrefixIndex + 1)..];
                }
            }
            else
            {
                parameterName = string.IsNullOrWhiteSpace(tempParameterName) ? "p" : tempParameterName;
            }

            // The VB compiler prefixes closure member names with $VB$Local_, remove that (#33150)
            if (parameterName.StartsWith("$VB$Local_", StringComparison.Ordinal))
            {
                parameterName = parameterName.Substring("$VB$Local_".Length);
            }

            if (!UseOldBehavior37465)
            {
                // In many databases, parameter names must start with a letter or underscore.
                // The same is true for C# variable names, from which we derive the parameter name, so in principle we shouldn't see an issue;
                // but just in case, prepend an underscore if the parameter name doesn't start with a letter or underscore.
                if (!char.IsLetter(parameterName[0]) && parameterName[0] != '_')
                {
                    parameterName = "_" + parameterName;
                }
            }

            if (UseOldBehavior37152)
            {
                // Uniquify the parameter name
                var originalParameterName = parameterName;
                for (var i = 0; _parameterNames.Contains(parameterName); i++)
                {
                    parameterName = originalParameterName + i;
                }
            }
            else
            {
                parameterName = Uniquifier.Uniquify(parameterName, _parameterNames, maxLength: int.MaxValue, uniquifier: _parameterNames.Count);
            }

            _parameterNames.Add(parameterName);
        }
        else
        {
            parameterName = string.Empty;
        }

Review comment (suggestion):
The code should check if parameterName is empty before accessing parameterName[0] to prevent a potential IndexOutOfRangeException. While unlikely (would require VB compiler to generate a field named exactly "$VB$Local_"), defensive programming suggests adding a length check: if (parameterName.Length > 0 && !char.IsLetter(parameterName[0]) && parameterName[0] != '_'). Note: This is a pre-existing theoretical issue that affects both old and new behavior paths, but becomes more visible with the new safety-net approach.
```suggestion
                if (parameterName.Length > 0 && !char.IsLetter(parameterName[0]) && parameterName[0] != '_')
```

Code after:
            {
                parameterName = tempParameterName ?? "p";

                var compilerPrefixIndex = parameterName.LastIndexOf('>');
                if (compilerPrefixIndex != -1)
                {
                    parameterName = parameterName[(compilerPrefixIndex + 1)..];
                }
            }
            else
            {
                parameterName = string.IsNullOrWhiteSpace(tempParameterName) ? "p" : tempParameterName;
            }

            // The VB compiler prefixes closure member names with $VB$Local_, remove that (#33150)
            if (parameterName.StartsWith("$VB$Local_", StringComparison.Ordinal))
            {
                parameterName = parameterName.Substring("$VB$Local_".Length);
            }

            if (!UseOldBehavior37465)
            {
                // In many databases, parameter names must start with a letter or underscore.
                // The same is true for C# variable names, from which we derive the parameter name, so in principle we shouldn't see an issue;
                // but just in case, prepend an underscore if the parameter name doesn't start with a letter or underscore.
                if (parameterName.Length > 0 && !char.IsLetter(parameterName[0]) && parameterName[0] != '_')
                {
                    parameterName = "_" + parameterName;
                }
            }

            if (UseOldBehavior37152)
            {
                // Uniquify the parameter name
                var originalParameterName = parameterName;
                for (var i = 0; _parameterNames.Contains(parameterName); i++)
                {
                    parameterName = originalParameterName + i;
                }
            }
            else
            {
                parameterName = Uniquifier.Uniquify(parameterName, _parameterNames, maxLength: int.MaxValue, uniquifier: _parameterNames.Count);
            }

            _parameterNames.Add(parameterName);
        }
        else
        {
            parameterName = string.Empty;
        }