Question

Say we have strings like:

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

I want to convert such strings into BinaryExpression objects similar to what we get by using:

  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 ) ) );

I have created the method below:

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" );
}

However, the above method does not work properly if, for example, we have strings like:

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'

What is the easiest way to make it more robust and reliable?

Answer 1

As Harsha pointed it out regex would make your task simple

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