antlr/antlr4 · error · Exception

This ATN simulator does not support clearing the DFA.

Error message

This ATN simulator does not support clearing the DFA.

What it means

ATNSimulator.ClearDFA() is a virtual method whose base implementation always throws, following the Java original's UnsupportedOperationException contract: only simulators that actually own an adaptable DFA cache (notably ParserATNSimulator) may clear it. Calling it on a simulator type that did not override the method is treated as an unsupported operation rather than a silent no-op, so mistakes surface immediately.

Source

Thrown at runtime/CSharp/src/Atn/ATNSimulator.cs:75

			this.sharedContextCache = sharedContextCache;
		}

		public abstract void Reset();

		/**
		 * Clear the DFA cache used by the current instance. Since the DFA cache may
		 * be shared by multiple ATN simulators, this method may affect the
		 * performance (but not accuracy) of other parsers which are being used
		 * concurrently.
		 *
		 * @throws UnsupportedOperationException if the current instance does not
		 * support clearing the DFA.
		 *
		 * @since 4.3
		 */
		public virtual void ClearDFA()
		{
			throw new Exception("This ATN simulator does not support clearing the DFA.");
		}

        public PredictionContextCache getSharedContextCache()
		{
			return sharedContextCache;
		}

		public PredictionContext getCachedContext(PredictionContext context)
		{
			if (sharedContextCache == null) return context;

			lock (sharedContextCache)
			{
				PredictionContext.IdentityHashMap visited =
					new PredictionContext.IdentityHashMap();
				return PredictionContext.GetCachedContext(context,
														  sharedContextCache,
														  visited);

View on GitHub (pinned to 7d5770395b)

Solutions

1. Call ClearDFA() only on a concrete ParserATNSimulator reference (or your own subclass that overrides ClearDFA)
2. If you own the subclass, override ClearDFA() and implement cache reset for your DFA storage
3. Instead of clearing, rebuild the simulator and its DFA array (recreate DFA[decision] entries), which works for any simulator type

Example fix

// before
ATNSimulator sim = GetSimulator();
sim.ClearDFA(); // throws Exception: does not support clearing the DFA

// after
if (sim is ParserATNSimulator parserSim)
    parserSim.ClearDFA();
else
    RebuildSimulator(); // recreate simulators and their DFA caches

static bool CanClearDfa(ATNSimulator sim) => sim is ParserATNSimulator;

try { sim.ClearDFA(); } catch (Exception ex) when (ex.Message.Contains("does not support clearing")) { /* rebuild simulators instead */ RebuildSimulators(); }

When it happens

Trigger: Calling ClearDFA() on an ATNSimulator subclass that does not override it (e.g. a custom simulator or an older-style instance), or on a simulator whose DFA cache design (shared/immutable DFA) makes clearing invalid. In the C# runtime ParserATNSimulator overrides it; the base and other subclasses do not.

Common situations: Porting Java code that called clearDFA() on a simulator variable statically typed as ATNSimulator; writing warmup/reset logic that clears caches after loading grammars; custom ATN simulator subclasses that forget to override ClearDFA.

Related errors

• Precedence predicates are not supported in lexers.
• cannot consume EOF
• EdgeMap of type {0} is supported yet.
• Only precedence DFAs may contain a precedence start state.
• listener cannot be null.

AI-assisted analysis of antlr/antlr4@7d5770395b (2026-08-14). Data as JSON: /api/errors/c667e69788e6dcdf. Report an issue: GitHub.