C# – difference between getting value from DataRow

cdatarowdatatable

Sample code:

    DataTable table = new DataTable();

    // ...
    // insert column to table

    table.Columns.Add("name");

    // ...
    // insert value to table

    foreach (DataRow row in table.Rows) {
         row["name"];
         row.Field<string>("name");     
    }

My question is:

Is there a difference between using row["name"] and row.Field<string>("name")? Of course, the second way cast value to some type, but is there another difference?
Which method is better to use?

Best Answer

See Remarks section, main differences described there:

The DataSet class represents null values with the Value instance of the DBNull class. A Language-Integrated Query (LINQ) expression that accessed a column with a null value would generate a InvalidCastException at run time. Additionally, DataSet does not support nullable types. The Field method provides support for accessing columns as nullable types. If the underlying value in the DataSet is Value, the returned nullable type will have a value of null.

If the value of the specified DataColumn is null and T is a reference type or nullable type, the return type will be null. The Field method will not return Value.

The Field method does not perform type conversions. If type conversion is required, you should first obtain the column value by using the Field method. The column value should then be converted to another type.

The last paragraph makes a point as I've often seen numbers stored as strings in database, therefore varchar to int conversion would be required on data retrieval, so in this case using DataColumn is better, e.g.:

int test = row.Field<int>("Test"); // InvalidCastException
int test = Convert.ToInt32(row["Test"]); // Works like a charm

DataRowExtensions.Field<T> Method (DataRow, String) first appeared in .NET 3.5 and it "provides strongly-typed access to each of the column values in the specified row. The Field method also supports nullable types."

Afaik, row["name"] returns object, row.Field<string>("name") returns a String. We shouldn't be comparing apples and pears, hence you should be asking what's better:

row["name"].ToString() vs row.Field<string>("name") and the answer is: they're same.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

Related Solutions

C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – the difference between const and readonly in C#

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