How to Combine two lambdas [duplicate]

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To complete Eric’s answer, using the new ExpressionVisitor introduced in .NET 4 rather than a custom rewriter:

internal class ParameterReplacer : ExpressionVisitor {
    private readonly ParameterExpression _parameter;

    protected override Expression VisitParameter(ParameterExpression node) {
        return base.VisitParameter(_parameter);
    }

    internal ParameterReplacer(ParameterExpression parameter) {
        _parameter = parameter;
    }
}

class Program {

    static void Main(string[] args) {
        Expression<Func<string, bool>> expr1 = s => s.Length == 5;
        Expression<Func<string, bool>> expr2 = s => s == "someString";
        var paramExpr = Expression.Parameter(typeof(string));
        var exprBody = Expression.Or(expr1.Body, expr2.Body);
        exprBody = (BinaryExpression) new ParameterReplacer(paramExpr).Visit(exprBody);
        var finalExpr = Expression.Lambda<Func<string, bool>>(exprBody, paramExpr);
    }

}

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