Question

假设我们有类似的字符串：

string s1 = "a < 5";
string s2 = "b >= 7";
string s3 = "c <= 8";
...

我想将这些字符串转换为BinaryExpression对象，类似于我们使用的字符：

  BinaryExpression b1 = Expression.MakeBinary( ExpressionType.LessThan, Expression.Parameter( typeof( int ), "a" ), Expression.Constant( 5, typeof( int ) ) );
  BinaryExpression b2 = Expression.MakeBinary( ExpressionType.GreaterThanOrEqual, Expression.Parameter( typeof( int ), "b" ), Expression.Constant( 7, typeof( int ) ) );
  BinaryExpression b3 = Expression.MakeBinary( ExpressionType.LessThanOrEqual, Expression.Parameter( typeof( int ), "c" ), Expression.Constant( 8, typeof( int ) ) );

我创建了以下方法：

BinaryExpression ConvertStringToBinaryExpression( string exp )
{
  string[] s = exp.Split( ' ' );
  string param = s[ 0 ];
  string comparison = s[ 1 ];
  int constant = int.Parse( s[ 2 ] );
  if ( comparison == "<" )
    return Expression.MakeBinary( ExpressionType.LessThan, Expression.Parameter( typeof ( int ), param ), Expression.Constant( constant, typeof ( int ) ) );
  else if ( comparison == "<=" )
    return Expression.MakeBinary( ExpressionType.LessThanOrEqual, Expression.Parameter( typeof ( int ), param ), Expression.Constant( constant, typeof ( int ) ) );
  else if ( comparison == ">" )
    return Expression.MakeBinary( ExpressionType.GreaterThan, Expression.Parameter( typeof ( int ), param ), Expression.Constant( constant, typeof ( int ) ) );
  else if ( comparison == ">=" )
    return Expression.MakeBinary( ExpressionType.GreaterThanOrEqual, Expression.Parameter( typeof ( int ), param ), Expression.Constant( constant, typeof ( int ) ) );
  else
    throw new ArgumentException( "Invalid expression.", "exp" );
}

但是，如果我们有类似的字符串，则上述方法无法正常工作：

string s4 = "a< 5"  // no space between 'a' and '<'
string s5 = "b>=9"  // no space at all
string s6 = "c <=7"  // no space betwen '<=' and '7'

使其更加强大和可靠的最简单方法是什么？

Answer 1

使用正则表达式匹配某些类型的可能表达式（如果您有可能的表达式的详尽且相当短的列表）。如果字符串与正则表达式不匹配，也要解析字符串，以检查是否有任何意外的字符。

编辑：正则表达式和解析将帮助您“清理”字符串，然后才能使用switch / if-else案例。

http://www.c-sharpcorner.com/UploadFile/prasad_1/RegExpPSD12062005021717AM/RegExpPSD.aspx

Answer 2

正如Harsha指出的那样regex会使你的任务变得简单

Match m=Regex.Match("var4 <= 433",@"(?'leftOperand'\w+)\s*(?'operator'(<|<=|>|>=))\s*(?'rightOperand'\w+)");
m.Groups["leftOperand"].Value;//the varaible or constant on left side of the operator
m.Groups["operator"].Value;//the operator
m.Groups["rightOperand"].Value;//the varaible or constant on right side of the operator

如何将字符串转换为BinaryExpression对象？

2 个答案: