替换列表的值<Class>

Question

I have two List<class> , List1 and List2 which contains multiple columns: RowNo , Value1 , Value2 , etc. as follows

List1

| RowNo | Value |
|-------|-------|
| 1     | 11    |
| 2     | 22    |
| 3     | 33    |
| 4     | 88    |

List2

| RowNo | Value |
|-------|-------|
| 1     | 44    |
| 2     | 55    |
| 3     | 66    |

I want to replace the value of element of List1 with the value of element of List2 if the RowNo matches.The output I want to generate is as follows

Desired result

| RowNo | Value |
|-------|-------|
| 1     | 44    |
| 2     | 55    |
| 3     | 66    |
| 4     | 88    |

Any Ideas or suggestions? How can I achieve this? What can be the best and efficient way to do this?

Answer 1

You can just use a loop to compare the values in List1 with List2 , and if a match is found, update the Value

foreach (var item in List1)
{
    var match = List2.FirstOrDefault(x => x.RowNo == item.RowNo);
    if (match != null)
    {
        item.Value = match.Value;
    }
}

Answer 2

Using Linq

List1.ForEach(l1 => l1.Value = (List2.FirstOrDefault(l2 => l2.RowNo == l1.RowNo) ?? l1).Value);

The Value property of l1 list element will be set to itself if no element will be found on the List2 list.

Full code

class MyClass
{
    public int RowNo { get; set; }
    public int Value { get; set; }
}

var List1 = new List<MyClass>()
{
    new MyClass(){RowNo = 1, Value = 11},
    new MyClass(){RowNo = 2, Value = 22},
    new MyClass(){RowNo = 3, Value = 33},
    new MyClass(){RowNo = 4, Value = 88},
};

var List2 = new List<MyClass>()
{
    new MyClass(){RowNo = 1, Value = 44},
    new MyClass(){RowNo = 2, Value = 55},
    new MyClass(){RowNo = 3, Value = 66}
};

List1.ForEach(l1 => l1.Value = (List2.FirstOrDefault(l2 => l2.RowNo == l1.RowNo) ?? l1).Value);

Answer 3

  List1.ForEach(x =>
        {
            var item = List2.FirstOrDefault(y => y.RowNo == x.RowNo);
            if (item != null)
            {
                x.Value = item.Value;
            }
        });

Answer 4

1. Put all data of list1 into a Dictionary (key is the RowNo).

2. Loop over list2 to update the Dictionary.

3. Convert the data of the Dictionary to a List .

It approaches an O(n) operation.

Answer 5

Use this extension method to achieve what you want:

public static class LinqExtentions
{
    public static void Project<T>(this IEnumerable<T> lst1, IEnumerable<T> lst2,
        Func<T, object> key, Action<T, T> action)
    {
        foreach (var item1 in lst1)
        {
            var item2 = lst2.FirstOrDefault(x => key(x).Equals(key(item1)));
            if (item2 != null)
            {
                action(item1, item2);
            }
        }
    }
}

then you can use it like this:

List1.Project(List2, x => x.RowNo, (y, z) => { y.Value = z.Value; });

What it does

is projecting one list over the other, then matching the key values in both ( RowNo in your example), when two items have the same key then the action supplied in the third parameter is applied, in this example, you want elements in the first list to have the same Value as elements in the second list, that's exactly what this delegate does:

(y, z) => { y.Value = z.Value; }

you can use this extension method to achieve the same requirement for any pair of lists:

1. Call Project on the list you want to change.
2. Pass the the list of the values you want to assign to the first list, as the first parameter.
3. Pass the key property as the second parameter.
4. The third parameter is the action you want to apply on your list.

Answer 6

You can loop over List1 and check if List2 contains a match then fill the result in a new list

List<YourClass> result = new List<YourClass>();
for (int i = 0; i < List1.Count; i++)
{
    YourClass resRowValue = List1[i];
    if (List2.Count > i && List2[i].RowValue.equals(resStrRowValue.RowValue)
        resStr.RowValue = List2[i].RowValue;

    result.Add(resRowValue);
}
//set the result to List1
List1 = result;

You can do this also using linq

List1 = List1.Select(x => {
    int i = List1.IndexOf(x);

    YourClass newValue = List2.FirstOrDefault(y => y.RowValue.Equals(x.RowValue));
    if (newValue != null)
        x.RowValue = newValue.RowValue;

    return x;
}).ToList();