（转）lambda表达式的解析（三） 类型转换表达式

接 上篇的常量表达式的转换，接下来要讲的是怎么产生操作运算表达式。

和C#通常意义上的3种操作符不同，我们的grammar把类型转换操作()从unary expression里拿出来独立成一个typecast_expression，也就是说要转换的操作符节点有4种。而且对于unary expression来说也有特别的地方，++,-- 这2个操作符不属于unray_operator下，grammar是把它们独立成pre_incr_decr_expression和post_incr_decr_expression 2块单独处理。还有就是在C#里as这个操作符是被定义成一元的，而我们的grammar则视它为二元操作符。

类型转换的操作很简单，这是grammar树：

在这里是做了一个int到int?的强类型转换，所以可以看type_ref里有一个qmark_opt，这表明是个nullable类型。

转换入口：

case "typecast_expression":
return ProcessConvertExpression(expNode);

case "typecast_expression": return ProcessConvertExpression(expNode);

具体转换代码：

private Expression ProcessConvertExpression(ParseTreeNode expNode)
{
var type = expNode.GetChild("typecast_parenthesized_expression").GetClrType();
var exp = ProcessExpression(expNode.GetChild("primary_expression"));
return Expression.Convert(exp, type);
}

private Expression ProcessConvertExpression(ParseTreeNode expNode) { var type = expNode.GetChild("typecast_parenthesized_expression").GetClrType(); var exp = ProcessExpression(expNode.GetChild("primary_expression")); return Expression.Convert(exp, type); }

可以看到这里为了取到要被强类型转换的对象，采用了递归调用ProcssExpression处理方式，而子节点里是一个literal，所以子节点会返回一个ConstantExpression。而GetClrType是treenode的扩展方法，它的代码在第一章里有提到。

public static Type GetClrType(this ParseTreeNode node)
{
if (node.HasChild("type_ref"))
{
var isNullable = node.GetDescendant("qmark_opt").FindTokenAndGetText() == "?";
var typeName = node.FindTokenAndGetText();
var type = ExpressionParser.GetType(typeName);
if (isNullable)
return typeof(Nullable<>).MakeGenericType(type);
return type;
}
return null;
}

public static Type GetClrType(this ParseTreeNode node) { if (node.HasChild("type_ref")) { var isNullable = node.GetDescendant("qmark_opt").FindTokenAndGetText() == "?"; var typeName = node.FindTokenAndGetText(); var type = ExpressionParser.GetType(typeName); if (isNullable) return typeof(Nullable<>).MakeGenericType(type); return type; } return null; }

就是根据type_ref子节点返回正确的clr类型。其中ExpressionParser.GetType方法如下：

internal static Type GetType(string typeName)
{
var type = Type.GetType(typeName);
if (type == null)
knownTypes.TryGetValue(typeName, out type);
return type;
}

internal static Type GetType(string typeName) { var type = Type.GetType(typeName); if (type == null) knownTypes.TryGetValue(typeName, out type); return type; }

就是通过typename得到Type，这里要注意的是int、char、string这些内建类型的名字并不是int、char、string，所以会有个knownTypes表来帮助返回正确的类型：

private static ConcurrentDictionary<string, Type> knownTypes = new ConcurrentDictionary<string, Type>();

#region BuiltinTypes
knownTypes["bool"] = typeof(bool);
knownTypes["sbyte"] = typeof(sbyte);
knownTypes["byte"] = typeof(byte);
knownTypes["short"] = typeof(short);
knownTypes["ushort"] = typeof(ushort);
knownTypes["int"] = typeof(int);
knownTypes["uint"] = typeof(uint);
knownTypes["long"] = typeof(long);
knownTypes["ulong"] = typeof(ulong);
knownTypes["float"] = typeof(float);
knownTypes["double"] = typeof(double);
knownTypes["char"] = typeof(char);
knownTypes["string"] = typeof(string);
knownTypes["decimal"] = typeof(decimal);
knownTypes["object"] = typeof(object);
knownTypes["void"] = typeof(void);
#endregion

private static ConcurrentDictionary<string, Type> knownTypes = new ConcurrentDictionary<string, Type>(); #region BuiltinTypes knownTypes["bool"] = typeof(bool); knownTypes["sbyte"] = typeof(sbyte); knownTypes["byte"] = typeof(byte); knownTypes["short"] = typeof(short); knownTypes["ushort"] = typeof(ushort); knownTypes["int"] = typeof(int); knownTypes["uint"] = typeof(uint); knownTypes["long"] = typeof(long); knownTypes["ulong"] = typeof(ulong); knownTypes["float"] = typeof(float); knownTypes["double"] = typeof(double); knownTypes["char"] = typeof(char); knownTypes["string"] = typeof(string); knownTypes["decimal"] = typeof(decimal); knownTypes["object"] = typeof(object); knownTypes["void"] = typeof(void); #endregion

正确的类型和正确的对象，创建个ConvertExpression也就非常简单了