MSBuild supports a specific set of conditions that can be applied wherever a Condition attribute is allowed. The following table explains those conditions.
Condition | Description |
|---|---|
'stringA' == 'stringB' | Evaluates to true if stringA equals stringB. For example: Condition="'$(CONFIG)'=='DEBUG'" Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values. |
'stringA' != 'stringB' | Evaluates to true if stringA is not equal to stringB. For example: Condition="'$(CONFIG)'!='DEBUG'" Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values. |
<, >, <=, >= | Evaluates the numeric values of the operands. Returns true if the relational evaluation is true. Operands must evaluate to a decimal or hexadecimal number. Hexadecimal numbers must begin with "0x".  NoteIn XML, the characters < and > must be escaped. The symbol < is represented as <. The symbol > is represented as >. |
Exists('stringA') | Evaluates to true if a file or folder with the name stringA exists. For example: Condition="!Exists('$(builtdir)')" Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values. |
HasTrailingSlash('stringA') | Evaluates to true if the specified string contains either a trailing backward slash (\) or forward slash (/) character. For example: Condition="!HasTrailingSlash('$(OutputPath)')" Single quotes are not required for simple alphanumeric strings or boolean values. However, single quotes are required for empty values. |
! | Evaluates to true if the operand evaluates to false. |
And | Evaluates to true if both operands evaluate to true. |
Or | Evaluates to true if at least one of the operands evaluates to true. |
() | Grouping mechanism that evaluates to true if expressions contained inside evaluate to true. |
Reference
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i.e.
Condition = "Exists('%(Outputs.RelativeDir)\*.dll') "
always evaluated to false even when there were .dlls in the Outputs folder
So I used this form instead: -
Condition="'$([System.IO.Directory]::GetFiles(%(Outputs.RelativeDir),*.dll).Length)'!='0'"
Given the following two lines of code I'd expect to get the same results, unfortunately that isn't the case.
<Files1 Include="$(OutputDir)\**\*_gac.txt" Condition=" Exists('$(OutputDir)\**\*_gac.txt') " />
<Files2 Include="$(OutputDir)\**\*_gac.txt" />
Files1 always returns an empty ItemGroup and given the explenation of Exists in Conditions I can't understand why.