diff --git a/csharp/ql/consistency-queries/PrimaryQlClass.ql b/csharp/ql/consistency-queries/PrimaryQlClass.ql
new file mode 100644
index 000000000000..eca49551c65f
--- /dev/null
+++ b/csharp/ql/consistency-queries/PrimaryQlClass.ql
@@ -0,0 +1,6 @@
+import csharp
+
+query predicate missingPrimaryQlClass(Element e) {
+  not exists(e.getAPrimaryQlClass()) and
+  e.fromSource()
+}
diff --git a/csharp/ql/lib/semmle/code/csharp/Generics.qll b/csharp/ql/lib/semmle/code/csharp/Generics.qll
index 9190523e3c08..ef3aced9cb9d 100644
--- a/csharp/ql/lib/semmle/code/csharp/Generics.qll
+++ b/csharp/ql/lib/semmle/code/csharp/Generics.qll
@@ -277,6 +277,8 @@ class TypeParameterConstraints extends Element, @type_parameter_constraints {
 
   /** Gets a textual representation of these constraints. */
   override string toString() { result = "where " + this.getTypeParameter().getName() + ": ..." }
+
+  override string getAPrimaryQlClass() { result = "TypeParameterConstraints" }
 }
 
 /**
diff --git a/csharp/ql/lib/semmle/code/csharp/Modifier.qll b/csharp/ql/lib/semmle/code/csharp/Modifier.qll
index 39652070af38..faae0c2c2e59 100644
--- a/csharp/ql/lib/semmle/code/csharp/Modifier.qll
+++ b/csharp/ql/lib/semmle/code/csharp/Modifier.qll
@@ -13,6 +13,8 @@ class Modifier extends Element, @modifier {
   predicate hasName(string name) { name = this.getName() }
 
   override string toString() { result = this.getName() }
+
+  override string getAPrimaryQlClass() { result = "Modifier" }
 }
 
 /**
@@ -20,4 +22,6 @@ class Modifier extends Element, @modifier {
  */
 class AccessModifier extends Modifier {
   AccessModifier() { this.hasName(["public", "private", "internal", "protected"]) }
+
+  override string getAPrimaryQlClass() { result = "AccessModifier" }
 }
diff --git a/csharp/ql/lib/semmle/code/csharp/Type.qll b/csharp/ql/lib/semmle/code/csharp/Type.qll
index 109c1df00c76..5aeaeb904d50 100644
--- a/csharp/ql/lib/semmle/code/csharp/Type.qll
+++ b/csharp/ql/lib/semmle/code/csharp/Type.qll
@@ -413,6 +413,8 @@ class VoidType extends DotNet::ValueOrRefType, Type, @void_type {
   final override string getUndecoratedName() { result = "Void" }
 
   override SystemNamespace getDeclaringNamespace() { any() }
+
+  override string getAPrimaryQlClass() { result = "VoidType" }
 }
 
 /**
@@ -454,6 +456,8 @@ class BoolType extends SimpleType, @bool_type {
   override string toStringWithTypes() { result = "bool" }
 
   override int getSize() { result = 1 }
+
+  override string getAPrimaryQlClass() { result = "BoolType" }
 }
 
 /**
@@ -467,6 +471,8 @@ class CharType extends SimpleType, @char_type {
   override int minValue() { result = 0 }
 
   override int maxValue() { result = 65535 }
+
+  override string getAPrimaryQlClass() { result = "CharType" }
 }
 
 /**
@@ -506,6 +512,8 @@ class SByteType extends SignedIntegralType, @sbyte_type {
   override int minValue() { result = -128 }
 
   override int maxValue() { result = 127 }
+
+  override string getAPrimaryQlClass() { result = "SByteType" }
 }
 
 /**
@@ -519,6 +527,8 @@ class ShortType extends SignedIntegralType, @short_type {
   override int minValue() { result = -32768 }
 
   override int maxValue() { result = 32767 }
+
+  override string getAPrimaryQlClass() { result = "ShortType" }
 }
 
 /**
@@ -532,6 +542,8 @@ class IntType extends SignedIntegralType, @int_type {
   override int minValue() { result = -2147483647 - 1 }
 
   override int maxValue() { result = 2147483647 }
+
+  override string getAPrimaryQlClass() { result = "IntType" }
 }
 
 /**
@@ -541,6 +553,8 @@ class LongType extends SignedIntegralType, @long_type {
   override string toStringWithTypes() { result = "long" }
 
   override int getSize() { result = 8 }
+
+  override string getAPrimaryQlClass() { result = "LongType" }
 }
 
 /**
@@ -552,6 +566,8 @@ class ByteType extends UnsignedIntegralType, @byte_type {
   override int getSize() { result = 1 }
 
   override int maxValue() { result = 255 }
+
+  override string getAPrimaryQlClass() { result = "ByteType" }
 }
 
 /**
@@ -563,6 +579,8 @@ class UShortType extends UnsignedIntegralType, @ushort_type {
   override int getSize() { result = 2 }
 
   override int maxValue() { result = 65535 }
+
+  override string getAPrimaryQlClass() { result = "UShortType" }
 }
 
 /**
@@ -572,6 +590,8 @@ class UIntType extends UnsignedIntegralType, @uint_type {
   override string toStringWithTypes() { result = "uint" }
 
   override int getSize() { result = 4 }
+
+  override string getAPrimaryQlClass() { result = "UIntType" }
 }
 
 /**
@@ -581,6 +601,8 @@ class ULongType extends UnsignedIntegralType, @ulong_type {
   override string toStringWithTypes() { result = "ulong" }
 
   override int getSize() { result = 8 }
+
+  override string getAPrimaryQlClass() { result = "ULongType" }
 }
 
 /**
@@ -597,6 +619,8 @@ class FloatType extends FloatingPointType, @float_type {
   override string toStringWithTypes() { result = "float" }
 
   override int getSize() { result = 4 }
+
+  override string getAPrimaryQlClass() { result = "FloatType" }
 }
 
 /**
@@ -606,6 +630,8 @@ class DoubleType extends FloatingPointType, @double_type {
   override string toStringWithTypes() { result = "double" }
 
   override int getSize() { result = 8 }
+
+  override string getAPrimaryQlClass() { result = "DoubleType" }
 }
 
 /**
@@ -615,6 +641,8 @@ class DecimalType extends SimpleType, @decimal_type {
   override string toStringWithTypes() { result = "decimal" }
 
   override int getSize() { result = 16 }
+
+  override string getAPrimaryQlClass() { result = "DecimalType" }
 }
 
 /**
@@ -775,6 +803,8 @@ class ObjectType extends Class {
   ObjectType() { this.hasQualifiedName("System.Object") }
 
   override string toStringWithTypes() { result = "object" }
+
+  override string getAPrimaryQlClass() { result = "ObjectType" }
 }
 
 /**
@@ -784,6 +814,8 @@ class StringType extends Class {
   StringType() { this.hasQualifiedName("System.String") }
 
   override string toStringWithTypes() { result = "string" }
+
+  override string getAPrimaryQlClass() { result = "StringType" }
 }
 
 /**
@@ -910,7 +942,9 @@ class FunctionPointerType extends Type, Parameterizable, @function_pointer_type
 /**
  * The `null` type. The type of the `null` literal.
  */
-class NullType extends RefType, @null_type { }
+class NullType extends RefType, @null_type {
+  override string getAPrimaryQlClass() { result = "NullType" }
+}
 
 /**
  * A nullable type, for example `int?`.
@@ -1124,6 +1158,8 @@ class TupleType extends ValueType, @tuple_type {
   final override predicate hasQualifiedName(string qualifier, string name) {
     this.getUnderlyingType().hasQualifiedName(qualifier, name)
   }
+
+  override string getAPrimaryQlClass() { result = "TupleType" }
 }
 
 /**
diff --git a/csharp/ql/lib/semmle/code/csharp/Variable.qll b/csharp/ql/lib/semmle/code/csharp/Variable.qll
index 6592320fdd79..aa66e59e7af3 100644
--- a/csharp/ql/lib/semmle/code/csharp/Variable.qll
+++ b/csharp/ql/lib/semmle/code/csharp/Variable.qll
@@ -338,6 +338,8 @@ class LocalVariable extends LocalScopeVariable, @local_variable {
   override Type getType() { localvars(this, _, _, _, getTypeRef(result), _) }
 
   override Location getALocation() { localvar_location(this, result) }
+
+  override string getAPrimaryQlClass() { result = "LocalVariable" }
 }
 
 /**
diff --git a/csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll b/csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll
index 8c53c7ea0c5b..1007d2cefe6c 100644
--- a/csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll
+++ b/csharp/ql/lib/semmle/code/csharp/exprs/Expr.qll
@@ -1084,6 +1084,8 @@ class DiscardExpr extends Expr, @discard_expr {
 
 private class UnknownExpr extends Expr, @unknown_expr {
   override string toString() { result = "Expression" }
+
+  override string getAPrimaryQlClass() { result = "UnknownExpr" }
 }
 
 /**
diff --git a/csharp/ql/lib/semmle/code/csharp/frameworks/System.qll b/csharp/ql/lib/semmle/code/csharp/frameworks/System.qll
index a14fa2268ba5..e7d8521a4244 100644
--- a/csharp/ql/lib/semmle/code/csharp/frameworks/System.qll
+++ b/csharp/ql/lib/semmle/code/csharp/frameworks/System.qll
@@ -112,6 +112,8 @@ class SystemBooleanStruct extends BoolType {
     result.getParameter(1).getType() instanceof BoolType and
     result.getReturnType() instanceof BoolType
   }
+
+  override string getAPrimaryQlClass() { result = "SystemBooleanStruct" }
 }
 
 /** Data flow for `System.Boolean`. */
@@ -1092,6 +1094,8 @@ class SystemIntPtrType extends ValueType {
     this = any(SystemNamespace n).getATypeDeclaration() and
     this.hasName("IntPtr")
   }
+
+  override string getAPrimaryQlClass() { result = "SystemIntPtrType" }
 }
 
 /** The `System.IDisposable` interface. */
diff --git a/csharp/ql/lib/semmle/code/dotnet/Element.qll b/csharp/ql/lib/semmle/code/dotnet/Element.qll
index bb6b2ebd906e..713261dcf361 100644
--- a/csharp/ql/lib/semmle/code/dotnet/Element.qll
+++ b/csharp/ql/lib/semmle/code/dotnet/Element.qll
@@ -43,8 +43,15 @@ class Element extends @dotnet_element {
 
   /**
    * Gets a comma-separated list of the names of the primary CodeQL classes to which this element belongs.
+   *
+   * If no primary class can be determined, the result is `"???"`.
    */
-  final string getPrimaryQlClasses() { result = concat(this.getAPrimaryQlClass(), ",") }
+  final string getPrimaryQlClasses() {
+    result = strictconcat(this.getAPrimaryQlClass(), ",")
+    or
+    not exists(this.getAPrimaryQlClass()) and
+    result = "???"
+  }
 
   /**
    * Gets the name of a primary CodeQL class to which this element belongs.
@@ -53,11 +60,12 @@ class Element extends @dotnet_element {
    * which they belong; for example, `AddExpr` is a primary class, but
    * `BinaryOperation` is not.
    *
-   * This predicate always has a result. If no primary class can be
-   * determined, the result is `"???"`. If multiple primary classes match,
-   * this predicate can have multiple results.
+   * If no primary classes match, this predicate has no result. If multiple
+   * primary classes match, this predicate can have multiple results.
+   *
+   * See also `getPrimaryQlClasses`, which is better to use in most cases.
    */
-  string getAPrimaryQlClass() { result = "???" }
+  string getAPrimaryQlClass() { none() }
 }
 
 /** An element that has a name. */
diff --git a/csharp/ql/lib/semmle/code/dotnet/Namespace.qll b/csharp/ql/lib/semmle/code/dotnet/Namespace.qll
index 324448728de2..9a0e9edac00d 100644
--- a/csharp/ql/lib/semmle/code/dotnet/Namespace.qll
+++ b/csharp/ql/lib/semmle/code/dotnet/Namespace.qll
@@ -39,6 +39,8 @@ class Namespace extends Declaration, @namespace {
   final override string getName() { namespaces(this, result) }
 
   final override string getUndecoratedName() { namespaces(this, result) }
+
+  override string getAPrimaryQlClass() { result = "Namespace" }
 }
 
 /** The global namespace. */
diff --git a/csharp/ql/lib/semmle/code/dotnet/Type.qll b/csharp/ql/lib/semmle/code/dotnet/Type.qll
index 81fefa965505..04b6cdaf6eb8 100644
--- a/csharp/ql/lib/semmle/code/dotnet/Type.qll
+++ b/csharp/ql/lib/semmle/code/dotnet/Type.qll
@@ -89,4 +89,6 @@ class ArrayType extends ValueOrRefType, @dotnet_array_type {
   final override string getLabel() { result = this.getElementType().getLabel() + "[]" }
 
   override string toStringWithTypes() { result = this.getElementType().toStringWithTypes() + "[]" }
+
+  override string getAPrimaryQlClass() { result = "ArrayType" }
 }