The expression language includes a grammar The expression language includes a Expression Grammatik to provide formatting groups, references and coalescing in C# strings.
Example
Below is a short example that uses these constructs to create a very flexible formatting string for a person.
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{id}'s full name: <{ln}, {fn}??withheld>
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This example includes the following rules:
The person's
idis always includes at the beginning (regardless of whether it is set to a non-null value).If both
lnandfnare set, they are both included and separated with a comma and a space.If only one of
lnorfnis set, that value is included.If neither
lnorfnis set, the word "withheld" is included instead.
Referencing data
References and embedded expressions are indicated with curly brackets:
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{A}
{A.B[2] + C('text')}
{A.B(C)}
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The angle bracket indicates a format group, where constant text is only included if one or both adjoining references are non-empty. The expression to the right of the ?? (coalesce) operator is used if the format group evaluates to empty. The ?? section is optional.
Reserved characters (<, ? and {) can be escaped with a backslash.
More examples
The following examples will try to illustrate the various paths that an evaluation of a formatting group can take.
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<{A}, {B}>
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A | B | Result |
|---|---|---|
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1 | 2 | 1, 2 |
1 | 1 | |
2 | 2 |
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<{A};{B};{C}: Message>
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A | B | C | Result |
|---|---|---|---|
| |||
1 | 2 | 3 | 1;2;3: Message |
1 | 2 | 1;2 | |
1 | 1 | ||
2 | 3 | 2;3: Message |
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<{A} items??empty>
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A | Result |
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1 | 1 items |
empty |
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<{A}, {B}.??<{C}??Nothing defined!>>
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A | B | C | Result |
|---|---|---|---|
1 | 2 | 3 | 1, 2 |
2 | 3 | 2 | |
1 | 3 | 1 | |
3 | 3 | ||
Nothing defined! |
Advanced Examples
A real-world example with several data fields can take a few more iterations to get them right. Let's take a look below.
...
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My name is <{LastName}, {FirstName} {MiddleInitial}.> and my phone number is <{MobNumber}??<{TelNumber}??<{FaxNumber}>>>.
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LastName | FirstName | MiddleInitial | MobNumber | TelNumber | FaxNumber | Result |
|---|---|---|---|---|---|---|
Doe | John | Q | 0765551111 | 0765552222 | 0765553333 | My name is Doe |
Doe | John | Q | 0765552222 | 0765553333 | My name is Doe | |
Doe | John | 0765552222 | 0765553333 | My name is Doe | ||
Doe | 0765553333 | My name is Doe and my phone number is 0765553333. | ||||
Doe | My name is Doe and my phone number is . | |||||
John | My name is John and my phone number is . | |||||
0765553333 | My name is and my phone number is 0765553333. | |||||
My name is and my phone number is . |
We can see from the sample input above that there are several instances where a phrase is included although all the data values are empty. The expression would work much better if the two formatting groups were merged.
...
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<My name is <{LastName}, {FirstName}, {MiddleInitial}.> and my phone number is <{MobNumber}??<{TelNumber}??<{FaxNumber}>>>.??No data>
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LastName | FirstName | MiddleInitial | MobNumber | TelNumber | FaxNumber | Result |
|---|---|---|---|---|---|---|
Doe | John | Q | 0765551111 | 0765552222 | 0765553333 | My name is Doe |
Doe | John | Q | 0765552222 | 0765553333 | My name is Doe | |
Doe | John | 0765552222 | 0765553333 | My name is Doe | ||
Doe | 0765553333 | My name is Doe and my phone number is 0765553333. | ||||
Doe | My name is Doe | |||||
John | My name is John | |||||
0765553333 | 0765553333. | |||||
No data |
As you can see, the fallbacks for missing data are now better, printing "No data" when absolutely nothing has been specified.
Writing tests
The following code illustrates how to test an expression using C# and NUnit with various inputs.
The test is written so that you can copy/paste most of it to your own solution and change the number of and the names of the parameters.
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