// Using this extension method you can get the value you wish
static string GetAttributeValue(this IEnumerable<(string key, string value)> attributes, string attribute) =>
    attributes.Where(x => x.key == attribute).First().value;

// Obviously this does not work if you have multiple values that are the same
// So, NOTE that this assumes that you want to get the first occurance

// Usage:
List<(string, string)> values = new List<(string, string)>()
{ ("asd", "asd"), ("asd1", "asd1") };

string asdValue = values.GetAttributeValue("asd1");
// Output: asd1