C# – List as datasource for DropDownList

.net-3.5cdatasourcedrop-down-menulist

I have a simple Class with ID and Name on it which I would like to link to a DropDownList but it seems that myDropDownList.DataTextField = "Name"; and myDropDownList.DataValueField = "ID"; are not accessible or available.

UPDATE: I am using winforms

public class Test
{
    public int ID { get; set; }
    public string Name { get; set; }
}

List<Test> myList = new List<Test>()
{
    // bla bla bla some entries bla bla bla you got the point.
};
myDropDownList.DataSource = myList;

I know I could override ToString but that would not help me with the value of each entry on the list.

Are there any other options to print the Name on the dropdownlist while having another property as the value (ie: printing Name while having the selected value or item as the ID) ?

Best Answer

For web based ASP.net

You need to specify the drop down lists datatextfield and datavaluefield properties.

MyDropDownList.DataSource = myList;
MyDropDownList.DataTextField="Name";
MyDropDownList.DataValueField="ID"; 
MyDropDownList.DataBind();

For win form

You need to specify the displaymember / valuemember properties.

Just noticed this is a winform application try this:

MyDropDownList.DataSource = myList;
MyDropDownList.DisplayMember = "Name";
MyDropDownList.ValueMember = "ID";

Caveats follow

Note that while this is perhaps the cleanest way to answer the question as asked, index is a rather weak component of the list API, and I can't remember the last time I used it in anger. It's been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index follow. It is probably worth initially taking a look at the documentation for it:

list.index(x[, start[, end]])
Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError if there is no such item.

The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.

Linear time-complexity in list length

An index call checks every element of the list in order, until it finds a match. If your list is long, and you don't know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000) is roughly five orders of magnitude faster than straight l.index(999_999), because the former only has to search 10 entries, while the latter searches a million:

>>> import timeit
>>> timeit.timeit('l.index(999_999)', setup='l = list(range(0, 1_000_000))', number=1000)
9.356267921015387
>>> timeit.timeit('l.index(999_999, 999_990, 1_000_000)', setup='l = list(range(0, 1_000_000))', number=1000)
0.0004404920036904514

Only returns the index of the first match to its argument

A call to index searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.

>>> [1, 1].index(1)
0
>>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
[0, 2]
>>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
>>> next(g)
0
>>> next(g)
2

Most places where I once would have used index, I now use a list comprehension or generator expression because they're more generalizable. So if you're considering reaching for index, take a look at these excellent Python features.

Throws if element not present in list

A call to index results in a ValueError if the item's not present.

>>> [1, 1].index(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: 2 is not in list

If the item might not be present in the list, you should either

Check for it first with item in my_list (clean, readable approach), or
Wrap the index call in a try/except block which catches ValueError (probably faster, at least when the list to search is long, and the item is usually present.)

Best Answer

For web based ASP.net

For win form

Related Solutions

Python – How to check if a list is empty

Python – Finding the index of an item in a list

Caveats follow

Linear time-complexity in list length

Only returns the index of the first match to its argument

Throws if element not present in list

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