use concurrent dictionary to replace lock

Author: WAcry

README.md

@@ -38,6 +38,18 @@ By default `LambdaParser` uses `ValueComparer` for values comparison. You can pr
 var valComparer = new ValueComparer() { NullComparison = ValueComparer.NullComparisonMode.Sql };
 var lambdaParser = new LambdaParser(valComparer); 
 ```
+### Caching Expressions
+
+The `UseCache` property determines whether the `LambdaParser` should cache parsed expressions. By default, `UseCache` is set to `true`, meaning expressions are cached to improve performance for repeated evaluations of the same expression. 
+
+Therefore, using a singleton instance of `LambdaParser` is recommended, rather than creating a new instance each time.
+
+You can disable caching by setting UseCache to false if you want to save memory, especially when evaluating a large number of unique expressions.
+
+```csharp
+var lambdaParser = new LambdaParser();
+lambdaParser.UseCache = false;
+```
 
 ## Who is using this?
 NReco.LambdaParser is in production use at [SeekTable.com](https://www.seektable.com/) and [PivotData microservice](https://www.nrecosite.com/pivotdata_service.aspx) (used for user-defined calculated cube members: formulas, custom formatting).

src/NReco.LambdaParser/Linq/LambdaParser.cs

@@ -13,6 +13,7 @@
 #endregion
 
 using System;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
 using System.Linq.Expressions;
@@ -36,9 +37,8 @@ public class LambdaParser {
 		static readonly string[] mulOps = new[] {"*", "/", "%" };
 		static readonly string[] addOps = new[] { "+", "-" };
 		static readonly string[] eqOps = new[] { "==", "!=", "<", ">", "<=", ">=" };
-		
-		readonly IDictionary<string, CompiledExpression> CachedExpressions = new Dictionary<string, CompiledExpression>();
-		readonly object _lock = new object();
+
+		readonly IDictionary<string, CompiledExpression> CachedExpressions = new ConcurrentDictionary<string, CompiledExpression>();
 
 		/// <summary>
 		/// Gets or sets whether LambdaParser should use the cache for parsed expressions.
@@ -124,9 +124,7 @@ public object Eval(string expr, IDictionary<string, object> vars) {
 		public object Eval(string expr, Func<string,object> getVarValue) {
 			CompiledExpression compiledExpr = null;
 			if (UseCache) {
-				lock (_lock) {
-					CachedExpressions.TryGetValue(expr, out compiledExpr);
-				}
+				CachedExpressions.TryGetValue(expr, out compiledExpr);
 			}
 
 			if (compiledExpr == null) {
@@ -136,11 +134,9 @@ public object Eval(string expr, Func<string,object> getVarValue) {
 				};
 				var lambdaExpr = Expression.Lambda(linqExpr, compiledExpr.Parameters);
 				compiledExpr.Lambda = lambdaExpr.Compile();
-				
+
 				if (UseCache)
-					lock (_lock) {
-						CachedExpressions[expr] = compiledExpr;
-					}
+					CachedExpressions[expr] = compiledExpr;
 			}
 
 			var valuesList = new List<object>();