Replacing a case-sensitive methods in an Expression Tree with a case-insensitive equivalents using QueryInterceptor. Very cool! This is runnable in LinqPad if you add the QueryInterceptor Nuget package

gistfile1.cs

void Main()
{
	
	var words = new []{"HELLO"}.AsQueryable().SetComparer(StringComparison.CurrentCultureIgnoreCase);
	
	words.Where (x => x.StartsWith("hel")).Dump(); 
	words.Where (x => x.Contains("ell")).Dump();
	words.Where (x => x.EndsWith("llo")).Dump();
	words.Where (x => x.Equals("hello")).Dump();
	words.Where (x => x == "hello").Dump();
	words.Where (x => x != "hello").Dump(); //dont want to match this one...
	
	 
}

public static class IQueryableCaseExtensions
{
	public static IQueryable<T> SetComparer<T>(this IQueryable<T> q, StringComparison sc){
		return q
			.InterceptWith(new SetComparerExpressionVisitor(sc));
	}
}

public class SetComparerExpressionVisitor : ExpressionVisitor
{
        readonly StringComparison _comparer;
        public SetComparerExpressionVisitor(StringComparison comparer)
        {
            _comparer = comparer;
        }

        protected override Expression VisitBinary(BinaryExpression node)
        {
            if (node.Left.Type == typeof (string) && node.Right.Type == typeof (string))
            {
                if (node.NodeType == ExpressionType.Equal)
                {

                    var exp = ((LambdaExpression) MakeExpression(s => s.Equals("asdf", _comparer))).Body;
                    exp = new ReplacingVisitor(((dynamic) exp).Arguments[0], ((dynamic) node).Left).Visit(exp);
                    exp = new ReplacingVisitor(((dynamic) exp).Object, ((dynamic) node).Right).Visit(exp);
                    return exp;
                }
                if (node.NodeType == ExpressionType.NotEqual)
                {
                    var exp = ((LambdaExpression) MakeExpression(s => !s.Equals("asdf", _comparer))).Body;
                    exp = new ReplacingVisitor(((dynamic) exp).Operand.Arguments[0], ((dynamic) node).Left).Visit(exp);
                    exp = new ReplacingVisitor(((dynamic) exp).Operand.Object, ((dynamic) node).Right).Visit(exp);
                    return exp;
                }
            }
            return base.VisitBinary(node);
        }

        protected override Expression VisitMethodCall(MethodCallExpression node)
        {
            if (node.Method.DeclaringType == typeof (string))
            {
                if (node.Method.Name == "Contains")
                {
                    var exp = ((LambdaExpression) MakeExpression(s => s.IndexOf("asdf", _comparer) > -1)).Body;
                    exp =
                        new ReplacingVisitor(((dynamic) exp).Left.Arguments[0], ((dynamic) node).Arguments[0]).Visit(exp);
                    exp = new ReplacingVisitor(((dynamic) exp).Left.Object, ((dynamic) node).Object).Visit(exp);
                    return exp;
                }
                if (node.Method.Name == "StartsWith")
                {
                    var exp = ((LambdaExpression) MakeExpression(s => s.IndexOf("asdf", _comparer) == 0)).Body;
                    exp =
                        new ReplacingVisitor(((dynamic) exp).Left.Arguments[0], ((dynamic) node).Arguments[0]).Visit(exp);
                    exp = new ReplacingVisitor(((dynamic) exp).Left.Object, ((dynamic) node).Object).Visit(exp);
                    return exp;
                }
                if (node.Method.Name == "EndsWith")
                {
                    var exp = ((LambdaExpression) MakeExpression(s => s.EndsWith("asdf", _comparer))).Body;
                    exp = new ReplacingVisitor(((dynamic) exp).Arguments[0], ((dynamic) node).Arguments[0]).Visit(exp);
                    exp = new ReplacingVisitor(((dynamic) exp).Object, ((dynamic) node).Object).Visit(exp);
                    return exp;
                }
                if (node.Method.Name == "Equals")
                {
                    var exp = ((LambdaExpression) MakeExpression(s => s.Equals("asdf", _comparer))).Body;
                    exp = new ReplacingVisitor(((dynamic) exp).Arguments[0], ((dynamic) node).Arguments[0]).Visit(exp);
                    exp = new ReplacingVisitor(((dynamic) exp).Object, ((dynamic) node).Object).Visit(exp);
                    return exp;
                }
            }

            return base.VisitMethodCall(node);
        }


        public Expression MakeExpression(Expression<Func<string, bool>> exp)
        {
            return exp;
        }
}


public class  ReplacingVisitor : ExpressionVisitor {
	Func<Expression, bool> match;
	Func<Expression, Expression> createReplacement;
 	public ReplacingVisitor(Expression from, Expression to){
		match = node => from == node;
		createReplacement = node => to;
	}
	
	public override Expression Visit(Expression node){
		if (match(node)) return createReplacement(node);
		return base.Visit(node);
	}
}

@davidfowl Thanks for your QueryInterceptor package!

thanks for this on many levels

About how to use this to provide case insensitive search fr Mvc.Jquery.Datatables: mcintyre321/mvc.jquery.datatables#44

I guess the string "asdf" is just a dummy string and can be anything?