Comparing generic value/string types

[Andrus]

I need to create generic table field level cache.
Table primary key (PrimaryKeyStru ctType) can be int, string or struct
containing int and string fields.
FieldName contains table field name to be cached.

Remove() should remove table row from cache. Row is identified by table
primary key.
Trying compile class causes error

Operator '==' cannot be applied to operands of type

'PrimaryKeyStru ctType' and 'PrimaryKeyStru ctType'

How to fix this ?

Andrus.

using System.Collecti ons.Generic;

class Cache<PrimaryKe yStructType{
struct Key {
public PrimaryKeyStruc tType PrimaryKey1Valu e;
public string FieldName;
}

Dictionary<Key, objectStore = new Dictionary<Key, object>();

/// <summary>
/// Removes table row from cache by removing all fields for this row
/// </summary>
/// <param name="id">Prima ry key value for row to be removed</param>
void Remove(PrimaryK eyStructType id) {
foreach (Key k in Store.Keys)
// Error 1 Operator '==' cannot be applied to operands of type
// 'PrimaryKeyStru ctType' and 'PrimaryKeyStru ctType'
if (k.PrimaryKey1V alue == id)
Store.Remove(k) ;
}
}

[Nicholas Paldino [.NET/C# MVP]]

Andrus,

In this case, you will have to create an equality (==) operator for the
type. In addition to doing that, you are going to want to consider
overriding the Equals and GetHashCode methods on the type as well.
--
- Nicholas Paldino [.NET/C# MVP]
- mv*@spam.guard. caspershouse.co m

"Andrus" <ko********@hot .eewrote in message
news:eG******** ******@TK2MSFTN GP02.phx.gbl...

>I need to create generic table field level cache.
Table primary key (PrimaryKeyStru ctType) can be int, string or struct
containing int and string fields.
FieldName contains table field name to be cached.

Remove() should remove table row from cache. Row is identified by table
primary key.
Trying compile class causes error

Operator '==' cannot be applied to operands of type

'PrimaryKeyStru ctType' and 'PrimaryKeyStru ctType'

How to fix this ?

Andrus.

using System.Collecti ons.Generic;

class Cache<PrimaryKe yStructType{
struct Key {
public PrimaryKeyStruc tType PrimaryKey1Valu e;
public string FieldName;
}

Dictionary<Key, objectStore = new Dictionary<Key, object>();

/// <summary>
/// Removes table row from cache by removing all fields for this row
/// </summary>
/// <param name="id">Prima ry key value for row to be removed</param>
void Remove(PrimaryK eyStructType id) {
foreach (Key k in Store.Keys)
// Error 1 Operator '==' cannot be applied to operands of type
// 'PrimaryKeyStru ctType' and 'PrimaryKeyStru ctType'
if (k.PrimaryKey1V alue == id)
Store.Remove(k) ;
}
}

[Andrus]

Nicholas,

this is generic type.
Ho to create equality operator and override Equals and GetHashCode for
generic type ?
This can be done only for convrete class.

Andrus.

In this case, you will have to create an equality (==) operator for the
type. In addition to doing that, you are going to want to consider
overriding the Equals and GetHashCode methods on the type as well.

[Jon Skeet [C# MVP]]

On Jul 2, 9:47 am, "Andrus" <kobrule...@hot .eewrote:

this is generic type.
Ho to create equality operator and override Equals and GetHashCode for
generic type ?
This can be done only for convrete class.

No, you can override both of these in generic types, just as normal.

The easiest way to implement them is to use
EqualityCompare r<T>.Default for each field (replacing the T
appropriately) to take IComparable etc into account.

Jon

[Andrus]

>this is generic type.

>Ho to create equality operator and override Equals and GetHashCode for
generic type ?
This can be done only for convrete class.

No, you can override both of these in generic types, just as normal.

The easiest way to implement them is to use
EqualityCompare r<T>.Default for each field (replacing the T
appropriately) to take IComparable etc into account.

Jon,

thank you.

If generic type can be int or string, how to implement this ?
Shound I use GetTypeCode() to determine is it int or string and then
cast variable to int or string to make comparison ?

Where to find example about this ?

Andrus.

[Jon Skeet [C# MVP]]

On Jul 2, 2:32 pm, "Andrus" <kobrule...@hot .eewrote:

If generic type can be int or string, how to implement this ?
Shound I use GetTypeCode() to determine is it int or string and then
cast variable to int or string to make comparison ?

No, there's no need. EqualityCompare r<T>.Default will do it all for
you. If you're in a generic type with type parameter of TFoo, just use
EqualityCompare r<TFoo>.Default .

Where to find example about this ?

If you could give a short but *complete* program, we could probably
implement it for you in a few lines.

Jon

[Jon Skeet [C# MVP]]

Andrus <ko********@hot .eewrote:

If you could give a short but *complete* program, we could probably
implement it for you in a few lines.

Here is complete program

Okay, a few problems:

1) Use the "T" prefix for type parameters - it makes it clearer what's
going on.

2) You shouldn't do this:

foreach (Key k in Store.Keys)
if (k.PrimaryKey1V alue == id)
Store.Remove(k) ;

as it will blow up when it comes back to iterating after removing a
key.

You *could* do:

foreach (Key k in Store.Keys)
{
if (k.PrimaryKey1V alue.Equals(id) )
{
Store.Remove(k) ;
break;
}
}

but that's not ideal and may not work depending on what you're
expecting it to do. The trouble is it's not clear what your key is
really meant to be. Should you be able to have multiple entries for the
same PrimaryStructTy pe?

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too