Correct naming for for switch/case pattern-matched captured variables

src/EFCore/Query/Internal/ExpressionTreeFuncletizer.cs

@@ -2080,20 +2080,22 @@ bool PreserveConvertNode(Expression expression)
 
         if (evaluateAsParameter)
         {
-            parameterName = tempParameterName ?? "p";
-
-            var compilerPrefixIndex = parameterName.LastIndexOf('>');
-            if (compilerPrefixIndex != -1)
-            {
-                parameterName = parameterName[(compilerPrefixIndex + 1)..];
-            }
+            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);
             }
 
+            // 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;
+            }
+
             parameterName = Uniquifier.Uniquify(parameterName, _parameterNames, maxLength: int.MaxValue, uniquifier: _parameterNames.Count);
 
             _parameterNames.Add(parameterName);
@@ -2147,11 +2149,13 @@ static Expression RemoveConvert(Expression expression)
                         switch (memberExpression.Member)
                         {
                             case FieldInfo fieldInfo:
-                                parameterName = parameterName is null ? fieldInfo.Name : $"{parameterName}_{fieldInfo.Name}";
+                                var name = SanitizeCompilerGeneratedName(fieldInfo.Name);
+                                parameterName = parameterName is null ? name : $"{parameterName}_{name}";
                                 return fieldInfo.GetValue(instanceValue);
 
                             case PropertyInfo propertyInfo:
-                                parameterName = parameterName is null ? propertyInfo.Name : $"{parameterName}_{propertyInfo.Name}";
+                                name = SanitizeCompilerGeneratedName(propertyInfo.Name);
+                                parameterName = parameterName is null ? name : $"{parameterName}_{name}";
                                 return propertyInfo.GetValue(instanceValue);
                         }
                     }
@@ -2166,7 +2170,7 @@ static Expression RemoveConvert(Expression expression)
                     return constantExpression.Value;
 
                 case MethodCallExpression methodCallExpression:
-                    parameterName = methodCallExpression.Method.Name;
+                    parameterName = SanitizeCompilerGeneratedName(methodCallExpression.Method.Name);
                     break;
 
                 case UnaryExpression { NodeType: ExpressionType.Convert or ExpressionType.ConvertChecked } unaryExpression
@@ -2190,6 +2194,25 @@ static Expression RemoveConvert(Expression expression)
                     exception);
             }
         }
+
+        static string SanitizeCompilerGeneratedName(string s)
+        {
+            // Compiler-generated field names intentionally contain illegal characters, specifically angle brackets <>.
+            // In cases where there's something within the angle brackets, that tends to be the original user-provided variable name
+            // (e.g. <PropertyName>k__BackingField). If we see angle brackets, extract that out, or it the angle brackets contain no
+            // content, strip them out entirely and take what comes after.
+            var closingBracket = s.IndexOf('>');
+            if (closingBracket == -1)
+            {
+                return s;
+            }
+
+            var openingBracket = s.IndexOf('<');
+
+            return openingBracket != -1 && openingBracket < closingBracket - 1
+                ? s[(openingBracket + 1)..closingBracket]
+                : s[(closingBracket + 1)..];
+        }
     }
 
     private Expression ConvertIfNeeded(Expression expression, Type type)

test/EFCore.Cosmos.FunctionalTests/Query/NorthwindMiscellaneousQueryCosmosTest.cs

@@ -2031,6 +2031,18 @@ FROM root c
 SELECT VALUE c
 FROM root c
 WHERE (c["$type"] = "Order")
+""");
+            });
+
+    public override Task Captured_variable_from_switch_case_pattern_matching(bool async)
+        => Fixture.NoSyncTest(
+            async, async a =>
+            {
+                await base.Captured_variable_from_switch_case_pattern_matching(a);
+
+                AssertSql(
+                    """
+ReadItem(None, ALFKI)
 """);
             });
 

test/EFCore.Specification.Tests/Query/NorthwindMiscellaneousQueryTestBase.cs

@@ -2993,6 +2993,23 @@ public virtual Task Parameter_extraction_can_throw_exception_from_user_code_2(bo
                     && o.OrderDate.Value.Year == dateFilter.Value.Year)));
     }
 
+    [ConditionalTheory, MemberData(nameof(IsAsyncData))]
+    public virtual async Task Captured_variable_from_switch_case_pattern_matching(bool async)
+    {
+        object customerIdAsObject = "ALFKI";
+
+        switch (customerIdAsObject)
+        {
+            case string customerId:
+            {
+                await AssertQuery(
+                    async,
+                    ss => ss.Set<Customer>().Where(c => c.CustomerID == customerId));
+                break;
+            }
+        }
+    }
+
     [ConditionalTheory, MemberData(nameof(IsAsyncData))]
     public virtual Task Subquery_member_pushdown_does_not_change_original_subquery_model(bool async)
         => AssertQuery(

test/EFCore.SqlServer.FunctionalTests/Query/NorthwindMiscellaneousQuerySqlServerTest.cs

@@ -3236,6 +3236,20 @@ FROM [Orders] AS [o]
 """);
     }
 
+    public override async Task Captured_variable_from_switch_case_pattern_matching(bool async)
+    {
+        await base.Captured_variable_from_switch_case_pattern_matching(async);
+
+        AssertSql(
+            """
+@customerId='ALFKI' (Size = 5) (DbType = StringFixedLength)
+
+SELECT [c].[CustomerID], [c].[Address], [c].[City], [c].[CompanyName], [c].[ContactName], [c].[ContactTitle], [c].[Country], [c].[Fax], [c].[Phone], [c].[PostalCode], [c].[Region]
+FROM [Customers] AS [c]
+WHERE [c].[CustomerID] = @customerId
+""");
+    }
+
     public override async Task Subquery_member_pushdown_does_not_change_original_subquery_model(bool async)
     {
         await base.Subquery_member_pushdown_does_not_change_original_subquery_model(async);