在Linq-to-Xml中处理空值和缺少列

Question

I have a user-supplied DataTable with the following information:

| Name      | Alias        |
|---------------------------
| Batman    | Dark Knight  |
| Superman  | Man of Steel |
| Spiderman | null         |

I need to convert this table to Xml to send to a stored procedure and get Xml similar to the following:

<Superheroes>
    <Superhero Name="Batman" Alias="Dark Knight"/>
    <Superhero Name="Superman" Alias="Man of Steel"/>
    <Superhero Name="Spiderman"/>
</Superheroes>

I want to use Linq-to-Xml (for learning purposes) to solve this problem. Here's what I have so far:

XDocument doc;

doc = new XDocument(
        new XElement("Superheroes",
        from row in table.AsEnumerable()
            select new XElement("Superhero",
                new XAttribute("Name", row.Field<string>("Name")),
                new XAttribute("Alias", row.Field<string>("Alias"))
)));

The problem is the Spiderman row. The query above doesn't work because of the null.

Questions

1. How do I handle the null Spiderman row in Linq-to-Xml?

2. If the user forgets to add the Alias row, I need to be able to handle that sitation. In that case, the Alias should either be (a) not in the Xml at all or (b) have empty values

Answer 1

You can write a helper function for this that will either return the XAttribute or null . Linq to XML will just ignore the null .

private XAttribute GetAlias(DataRow row)
{
    if(row.Field<string>("Alias") != null)
    {
        return new XAttribute("Alias", row.Field<string>("Alias"));
    }
    return null;
}

And use it like this.

doc = new XDocument(
    new XElement("Superheroes",
    from row in table.AsEnumerable()
        select new XElement("Superhero",
            new XAttribute("Name", row.Field<string>("Name")),
            GetAlias(row)
)));

This should also work if the Alias column is missing because the Field<T>(string) extension method should return null if the column is missing.

Alternatively if you just want to have the attribute with an empty string when the column is missing or the value is null you can do the following.

doc = new XDocument(
    new XElement("Superheroes",
    from row in table.AsEnumerable()
        select new XElement("Superhero",
            new XAttribute("Name", row.Field<string>("Name")),
            new XAttribute("Alias", row.Field<string>("Alias") ?? string.Empty)
)));

Answer 2

另一种方法是像在第一个答案中那样使用三元运算符： 具有三元运算符的示例

Answer 3

One more solution if attribute names should be same as column captions. You can add only those attributes, which have not null values:

doc = new XDocument(
         new XElement("Superheroes",
            from row in table.AsEnumerable()
            select new XElement("Superhero",
                from column in table.Columns.Cast<DataColumn>()
                let value = row.Field<string>(column)
                where value != null // just filter out nulls
                select new XAttribute(column.Caption, value)
        )));

BTW Spiderman is not that good as Batman or Superman

---

Actually you can create following extension, which will build xml from any DataTable based on table name and column names (it requires PluralizationService from System.Data.Entity.Design assembly):

public static class Extensions
{
    public static XDocument ToXml(this DataTable table)
    {
        if (String.IsNullOrEmpty(table.TableName))
           throw new ArgumentException("Table name is required");

        var pluralizationService = PluralizationService
                .CreateService(new CultureInfo("en-US"));
        string elementName = pluralizationService.Singularize(table.TableName);

        return new XDocument(
            new XElement(table.TableName,
                from row in table.AsEnumerable()
                select new XElement(elementName,
                    from column in table.Columns.Cast<DataColumn>()
                    let value = row.Field<string>(column)
                    where value != null
                    select new XAttribute(column.Caption, value)
                    )
                )
            );
    }
}

Usage is simple:

DataTable table = new DataTable("Superheroes"); // name is required
table.Columns.Add("Name");
table.Columns.Add("Alias");
table.Rows.Add("Batman", "Dark Knight");
table.Rows.Add("Superman", "Man Of Steel");
table.Rows.Add("Spiderman", null);
var doc = table.ToXml();

And result is pretty nice

<Superheroes>
  <Superhero Name="Batman" Alias="Dark Knight" />
  <Superhero Name="Superman" Alias="Man Of Steel" />
  <Superhero Name="Spiderman" />
</Superheroes>

Answer 4

You can extend your query with inline if statement (using ?: operator ) and check if Alias is set in current row:

doc = new XDocument(
        new XElement("Superheroes",
        from row in table.AsEnumerable()
        select new XElement("Superhero",
            new XAttribute("Name", row.Field<string>("Name")),
            (row.Field<string>("Alias") != null ? new XAttribute("Alias", row.Field<string>("Alias")) : null))
));

Returning null does not change generated XML at all, so doc contains exactly what you want it to contain:

<Superheroes>
  <Superhero Name="Batman" Alias="Dark Knight" />
  <Superhero Name="Superman" Alias="Man of Steel" />
  <Superhero Name="Spiderman" />
</Superheroes>