C# Square brackets for accessing Objects with String

Question

我不明白这个方括号，我必须创建什么，这个代码行正在工作，仅用于测试。

        filetype = dataBandREP["VRZ.Parent.SIGNATUR"].ToString();

Answer 1

C# can overload operator, including square brackets.

public class Example
{
    public string this[string s] // square bracket operator with string argument
    {
        get 
        {
            return somethingToReturnString;
        }
        set
        {
            somethingToSetString = value;
        }
    }

    public string this[int i] // square bracket operator with int argument
    {
        get 
        {
            return somethingToReturnInt;
        }
        set
        {
            somethingToSetInt = value;
        }
    }
}

Answer 2

The square brackets mean you are referencing a collection and within that collection, you want "VRZ.Parent.SIGNATUR". [] is an indexer with a string as argument.

Answer 3

This code: filetype = dataBandREP["VRZ.Parent.SIGNATUR"].ToString(); uses the indexer of some sort of collection, referring to an object, then converting it to its string format in order to populate the filetype variable.

A very common usage of this is with arrays and DataColumn collections. String indexers allow you to fetch a value based on, say, a column header, instead of knowing what integral index in the DataTable.Columns collection is associated with the column.

You're getting different behavior when debugging because the validity of "VRZ.Parent.SIGNATUR" as a string indexer has changed, probably because of a change in the actual data source. That's assuming that you're getting a null reference or similar. Without knowing what exception you're getting, we can't help much more than that.

Answer 4

Below syntax also works.. my class is derived from a ReadOnlyDictionary of some type "AttributeValue" and it exports double values. It gives the user an error box when the value is not found.

public abstract class AttribDoubleReader: IReadOnlyDictionary<string, AttributeValue>
{
 public double this[string key] => GetValue(key);

 private double GetValue(string key)
 {
    if (TryGetValue(key, out AttributeValue value))
    {
        return value.AsDouble();
    }
    else
    {
        MessageBox.Show("ERROR: KEY "+ key + " NOT FOUND.");
        throw new KeyNotFoundException();
    }

 }
}