C#: Add new class AssignableDefinitionNode to the data-flow library

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -554,15 +554,10 @@ class SsaDefinitionNode extends Node, TSsaDefinitionNode {
 
 private module ParameterNodes {
   /**
-   * Holds if SSA definition node `node` is an entry definition for parameter `p`.
+   * Holds if definition node `node` is an entry definition for parameter `p`.
    */
-  predicate explicitParameterNode(SsaDefinitionNode node, Parameter p) {
-    exists(Ssa::ExplicitDefinition def, AssignableDefinitions::ImplicitParameterDefinition pdef |
-      node = TSsaDefinitionNode(def)
-    |
-      pdef = def.getADefinition() and
-      p = pdef.getParameter()
-    )
+  predicate explicitParameterNode(AssignableDefinitionNode node, Parameter p) {
+    p = node.getDefinition().(AssignableDefinitions::ImplicitParameterDefinition).getParameter()
   }
 
   /**
@@ -1126,13 +1121,14 @@ private module OutNodes {
    * A data flow node that reads a value returned by a callable using an
    * `out` or `ref` parameter.
    */
-  class ParamOutNode extends OutNode, SsaDefinitionNode {
+  class ParamOutNode extends OutNode, AssignableDefinitionNode {
     private AssignableDefinitions::OutRefDefinition outRefDef;
+    private ControlFlow::Node cfn;
 
-    ParamOutNode() { outRefDef = this.getDefinition().(Ssa::ExplicitDefinition).getADefinition() }
+    ParamOutNode() { outRefDef = this.getDefinitionAtNode(cfn) }
 
     override DataFlowCall getCall(ReturnKind kind) {
-      result = csharpCall(_, this.getDefinition().getControlFlowNode()) and
+      result = csharpCall(_, cfn) and
       exists(Parameter p |
         p.getSourceDeclaration().getPosition() = kind.(OutRefReturnKind).getPosition() and
         outRefDef.getTargetAccess() = result.getExpr().(Call).getArgumentForParameter(p)
@@ -1458,10 +1454,8 @@ class CastNode extends Node {
   CastNode() {
     this.asExpr() instanceof Cast
     or
-    exists(Ssa::ExplicitDefinition def |
-      def = this.(SsaDefinitionNode).getDefinition() and
-      def.getADefinition() instanceof AssignableDefinitions::PatternDefinition
-    )
+    this.(AssignableDefinitionNode).getDefinition() instanceof
+      AssignableDefinitions::PatternDefinition
     or
     readStep(_, _, this)
     or

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPublic.qll

@@ -19,13 +19,23 @@ class Node extends TNode {
    * if any.
    */
   Expr asExprAtNode(ControlFlow::Nodes::ElementNode cfn) {
-    this = TExprNode(cfn) and
-    result = cfn.getElement()
+    result = this.(ExprNode).getExprAtNode(cfn)
   }
 
   /** Gets the parameter corresponding to this node, if any. */
   DotNet::Parameter asParameter() { result = this.(ParameterNode).getParameter() }
 
+  /** Gets the definition corresponding to this node, if any. */
+  AssignableDefinition asDefinition() { result = this.asDefinitionAtNode(_) }
+
+  /**
+   * Gets the definition corresponding to this node, at control flow node `cfn`,
+   * if any.
+   */
+  AssignableDefinition asDefinitionAtNode(ControlFlow::Node cfn) {
+    result = this.(AssignableDefinitionNode).getDefinitionAtNode(cfn)
+  }
+
   /** Gets the type of this node. */
   cached
   DotNet::Type getType() { none() }
@@ -140,6 +150,22 @@ class ParameterNode extends Node {
   predicate isParameterOf(DataFlowCallable c, int i) { none() }
 }
 
+/** A definition, viewed as a node in a data flow graph. */
+class AssignableDefinitionNode extends Node, TSsaDefinitionNode {
+  private Ssa::ExplicitDefinition edef;
+
+  AssignableDefinitionNode() { this = TSsaDefinitionNode(edef) }
+
+  /** Gets the underlying definition. */
+  AssignableDefinition getDefinition() { result = this.getDefinitionAtNode(_) }
+
+  /** Gets the underlying definition, at control flow node `cfn`, if any. */
+  AssignableDefinition getDefinitionAtNode(ControlFlow::Node cfn) {
+    result = edef.getADefinition() and
+    cfn = edef.getControlFlowNode()
+  }
+}
+
 /** Gets a node corresponding to expression `e`. */
 ExprNode exprNode(DotNet::Expr e) { result.getExpr() = e }
 
@@ -148,6 +174,11 @@ ExprNode exprNode(DotNet::Expr e) { result.getExpr() = e }
  */
 ParameterNode parameterNode(DotNet::Parameter p) { result.getParameter() = p }
 
+/** Gets a node corresponding to the definition `def`. */
+AssignableDefinitionNode assignableDefinitionNode(AssignableDefinition def) {
+  result.getDefinition() = def
+}
+
 /**
  * Holds if data flows from `nodeFrom` to `nodeTo` in exactly one local
  * (intra-procedural) step.