Resolve errors and warnings when throwing and catching exceptions

This article covers compiler diagnostics related to throwing and catching exceptions. The following warnings are generated when an exception filter is a constant false:

• CS8359: Filter expression is a constant 'false', consider removing the catch clause
• CS8360: Filter expression is a constant 'false', consider removing the try-catch block

Remove a constant false exception filter

• CS8359: Filter expression is a constant 'false', consider removing the catch clause
• CS8360: Filter expression is a constant 'false', consider removing the try-catch block

A catch clause with a filter that is always false never handles an exception. The compiler reports CS8360 when the try statement has exactly one catch clause and no finally clause. In that case, the entire try-catch statement is ineffective. The compiler reports CS8359 when the try statement has another catch clause or a finally clause.

The following example produces CS8359 on the filter because another catch clause can handle an exception. The compiler also reports unreachable code in the constant-false handler:

using System;
using System.IO;

try
{
    Console.WriteLine(File.ReadAllText("data.txt"));
}
catch (InvalidOperationException ex) when (false) // CS8359
{
    Console.Error.WriteLine(ex.Message);
}
catch (IOException ex)
{
    Console.Error.WriteLine(ex.Message);
}

Remove the constant-false catch clause. If the clause should handle some exceptions, replace the constant expression with a meaningful runtime condition.

The following example produces CS8360 on the filter because the constant-false filter is on the only catch clause and the statement has no finally clause. The compiler also reports unreachable code in the handler:

using System;
using System.IO;

try
{
    Console.WriteLine(File.ReadAllText("data.txt"));
}
catch (InvalidOperationException ex) when (false) // CS8360
{
    Console.Error.WriteLine(ex.Message);
}

Remove the try-catch statement while preserving the statements from the try block. If the handler is required conditionally, replace the constant filter with a meaningful runtime condition.

For more information, see Advantages of exception filters.